Preparation and electrochemical characterization of cation- and anion-exchange/polyaniline composite membranes

Composite membranes were prepared by chemical polymerization of a thin layer of polyaniline (PANI) in the presence of a high oxidant concentration on a single face of a sulfonated cation-exchange membrane (CEM) and quaternary aminated anion-exchange membrane (AEM). IR and SEM studies for both types of membranes confirmed PANI loading on the ion-exchange membranes. PANI composite ion-exchange membranes were characterized as a function of the polymerization time by ion-exchange capacity, coating density, and membrane conductance measurements. Membrane potential measurements were performed in various electrolyte solutions in order to observe the selectivity of these membranes for different types of counterions. Membrane potential data in conjunction with membrane conductance data was interpreted on the basis of frictional considerations between membrane matrix and solute. Electrodialysis experiments, using PANI composite ion-exchange membranes with 4 h polymerization time, were performed in single and mixed electrolyte solutions for observing electromigration of solute across PANI composite ion-exchange membranes. Relative dialytic rates of Na(2)SO(4), CaCl(2), and CuCl(2) were estimated with reference to NaCl on the basis of electrodialysis experiments and it was concluded that it is possible to separate different electrolytes using PANI composite ion-exchange membranes.     

Interfacially formed poly(N,N-dimethylaminoethyl methacrylate)/polysulfone composite membranes for CO2/N2 separation

Interfacially formed poly(N,N-dimethylaminoethyl methacrylate)/polysulfone (PDMAEMA/PSF) composite membranes were developed for CO₂/N₂ separation. A layer of PDMAEMA was deposited on a microporous PSF substrate by the solution coating technique, followed by crosslinking with p-xylylene dichloride (XDC) at the interface between the PDMAEMA solid layer and the crosslinking solution. The hydrophilicity and surface free energy of the membranes were analyzed by contact angle measurements with different probe liquids. The permselectivity of the membrane was shown to be affected by the PDMAEMA deposition time, interfacial crosslinking reaction time, and the PDMAEMA and XDC concentrations in the polymer coating solution and the crosslinking solution, respectively. The composite membrane showed a CO₂ permeance of 85 GPU and a CO₂/N₂ ideal separation factor of 50 at 23 °C and 0.41 MPa of CO₂ feed pressure.     

Ionic transport phenomenon across sol–gel derived organic–inorganic composite mono-valent cation selective membranes

Organic–inorganic composite mono-valent cation selective membranes (MCSMs) were prepared by sol–gel under acidic conditions, in which sulfonic acid groups were introduced at the inorganic segment. Studies on physicochemical and electrochemical properties revealed their excellent mechanical, thermal, and oxidative stabilities, high conductivity, ion-exchange capacity, permselectivity for mono-valent cations, ionic diffusion and water transport number. These properties suggested the suitability of MCSMs, especially Si-65%, for electro-separation of Na⁺ from Ca²⁺, Mg²⁺, and Fe³⁺. The effect of electrolyte solution on the characteristics of the current–voltage (i–v) curve in MCSM was studied based on the concentration polarization. Electro-transport of different ions in terms of plateau length and concentration profiles for different ions in the solution phase, diffusion boundary layer and membrane phase were presented. Information obtained from i–v curve analysis were validated by electrodialysis (ED) experiments for individual or mixed electrolyte solutions. Electro-transport efficiency and separation factor of different ions for MCSM and Nafion117 (N117) membranes were compared, which suggested suitability of MCSMs for separating cations.     

Preparation of water and alkali durable porous glass membrane coated on porous alumina tubing by sol-gel method

Composite porous glass membranes were prepared by the sol-gel method. A thin porous glass layer, about 2 μm thick, was coated on the surface of the porous ceramic tubing (Al₂O₃:99.9 wt.%, pore diameter: 200 nm). The composition of the porous glass layer of the composite membrane was SiO₂-ZrO₂. Considering from the fact that the desalination ratio of the feed aqueous NaCl solution (NaCl 0.5 wt.%) was about 90% by use of these membranes, they were defect-free. The best composition of the porous glass layer was 70 SiO₂-30 ZrO₂ from the standpoint of preparing membranes. These membranes had a large water and alkali durability. These membranes can be expected to apply to recovering dyes and paints from organic solvents and to be used as a gas separation membrane.     

Preparation and characterization of N,O-carboxymethyl chitosan (NOCC)/polysulfone (PS) composite nanofiltration membranes

N,O-carboxymethyl chitosan (NOCC) composite nanofiltration membranes having a polysulfone (PS) UF membrane as the substrate were prepared using a method of coating and cross-linking, in which a glutaraldehyde (GA) aqueous solution was used as the cross-linking agent. Attenuated total reflection infrared spectroscopy (ATR-IR) was employed to characterize the resulting membrane. The effects of the composition of the casting solution of the active layer, the concentration of the cross-linking agent, and the membrane preparation techniques on the performance of the composite membrane were investigated. At 13–15 °C and 0.40 MPa the rejections of the resulting membrane to Na₂SO₄ and NaCl solutions (1000 mg L⁻¹) were 92.7 and 30.2%, respectively, and the permeate fluxes were 3.0 and 5.1 kg m⁻² h⁻¹, respectively. The rejection of this kind of membrane to the electrolyte solutions decreased in the order of Na₂SO₄, NaCl, MgSO₄, and MgCl₂. This suggests that the membrane active layer acquires a negative surface charge distribution by the adsorption of anions from the electrolyte solution and this charge distribution mainly determines the membrane performance.     

Pd encapsulated and nanopore hollow fiber membranes: Synthesis and permeation studies

New types of supported Pd membranes were developed for high temperature H₂ separation. Sequential combinations of boehmite sol slip casting and film coating, and electroless plating (ELP) steps were designed to synthesize “Pd encapsulated” and “Pd nanopore” membranes supported on -Al₂O₃ hollow fibers. The permeation characteristics (flux, permselectivity) of a series of unaged and aged encapsulated and nanopore membranes with different Pd loadings were compared to those of a conventional 1 μm Pd/4 μm γ-Al₂O₃/-Al₂O₃ hollow fiber membrane. The unaged encapsulated membrane exhibited good performance with ideal H₂/N₂ separation factors of 3000–8000 and H₂ flux 0.4 mol/m² s at 370 °C and a transmembrane pressure gradient of 4 × 10⁵ Pa. The unaged Pd nanopore membranes had a lower initial flux and permselectivity, but exhibited superior performance with extended use (200 h). At the same conditions the unaged 2.6 μm Pd nanopore membrane had a H₂ flux of 0.16 mol/m² s and separation factor of 500 and the unaged 0.6 μm Pd nanopore membrane had a H₂ flux of 0.25 mol/m² s and separation factor of 50. Both nanopore membranes stabilized after 40 h of operation, in contrast to a continued deterioration of the permselectivity for the other membranes. An analysis of the permeation data reveals a combination of Knudsen and convective transport through membrane defects. A phenomenological, qualitative model of the synthesis and resulting structure of the encapsulated and nanopore membranes is presented to explain the permeation results.     

Electrodialysis of calcium and carbonate high concentration solutions and impact on composition in cations of membrane fouling

Fouling, which is the accumulation of undesired solid materials at the phase interfaces of permselective membranes, is one of the major problems in electrodialysis. The objectives of the present work were to investigate the effect of the composition in calcium and carbonate of a model solution to be treated by conventional electrodialysis on their migration kinetics and the composition in cations of the membrane fouling. In the absence of sodium carbonate in the solution, no fouling was visually observed on anion-exchange membranes (AEM) and fouling was observed only at 1600 mg/L CaCl2 on cation-exchange membrane (CEM), while at only 800 mg/L CaCl2 with sodium carbonate, a deposit was observed on both membranes. This difference could be explained by the fact that carbonate has a high buffer capacity, and the time to reach pH 4.0 was then longer than the one without carbonate. Consequently, the migration of the ionic species was carried out over a longer period of time during ED treatment with sodium carbonate addition and in extent the demineralization rates were higher: 43 vs 86%. For treatment with sodium carbonate and 1600 mg/L CaCl2, the higher migration during ED treatment, increased the concentration of calcium, from 14.24 to 93.38 mg/g dry membrane and from 0.74 to 10.27 mg/g dry membrane for CEM and AEM, respectively. Due to the basic pH on the side of the membrane in contact with the NaCl solution, the calcium would precipitate to form calcium hydroxide on CEM while the calcium migrated through the CEM was blocked by the AEM where it formed another fouling.     

Surface modification of mesoporous membranes by fluoro-silane coupling reagent for CO2 separation

The potential of hybrid organic–inorganic membranes for separating organic molecules from air, based on solubility selective mechanism, was evaluated. Alumina and titana membranes with average pore size near 4 nm were surface modified using trimethoxysilane fluorinated coupling reagent. The permeabilities to helium, nitrogen, methane, ethane, propane, butane and carbon dioxide were evaluated at feed pressures lying between (1.5 × 10⁵ and 3.5 × 10⁵ Pa) 1.5 and 3.5 bar and permeate outlet near 1 × 10⁵ Pa (1 bar). The permeabilities of the grafted membranes generally decreased by about two to three orders of magnitude compared with the untreated membranes. The CO₂/N₂ permselectivity increased significantly in the case of the TiO₂ grafted membrane. The membranes performances were compared and the TiO₂ grafted membrane exhibits higher permselectivity and permeability, so that, it is a good candidate for CO₂ to N₂ separation and CO₂ to hydrocarbon separation.     

Surface modification of γ-alumina membranes by silane coupling for CO2 separation

Top layers of γ-Al₂O₃ composite membranes have been modified by the silane coupling technique using phenyltriethoxysilane for improving the separation factor of CO₂ to N₂. The separation efficiency of the modified membranes was strongly dependent upon the hydroxylation tendency of the support materials and the amount of the special functional group (i.e. phenyl radical) which was coupled onto a top layer. The separation factor through the TiO₂ supported γ-Al₂O₃ membrane was found to be fairly enhanced by silane coupling, but in case of the -Al₂O₃ supported membrane was not. The CO₂/N₂ separation factor through the modified γ-Al₂O₃/TiO₂ composite membrane is 1.7 at 90°C and ΔP = 2 × 10⁵ Pa for the binary mixture containing 50 vol% CO₂. The separation factor is proportional to the CO₂ concentration in the gas mixture, and the modified membrane is stable up to 100°C. The main mechanism of the CO₂ transport through the modified γ-Al₂O₃ layer is known to be a surface diffusion.     

Comparative studies on electrochemical characterization of homogeneous and heterogeneous type of ion-exchange membranes

Interpolymer films of poly-ethylene and styrene-divinyl benzene copolymer were subjected to chlorosulfonation or chloromethylation then amination for the preparation of homogeneous type of cation- or anion-exchange membranes, respectively. Heterogeneous types of ion-exchange membranes were prepared from polyvinyl chloride (PVC) as binder and ion-exchange resin powder in tetrahydrofuran solvent. Membrane potential and conductance measurements have been carried out in NaCl(aq), CuCl₂(aq) and AlCl₃(aq) solutions at different concentrations to investigate the relationship between concentration of fixed charges and electrochemical properties of these membranes. On the basis of the micro-heterogeneous model, describing the micro-structure of the membrane material, the counter-ion diffusion coefficients were estimated. Membrane conductance data, along with values of concentration of fixed ionic site in the membrane, were used for the estimation of the tortuosity factor and salt permeability employing non-equilibrium thermodynamic principles. It was concluded that electrochemical transport properties of homogeneous type of ion-exchange membranes are superior to those for heterogeneous type of ion-exchange membranes. However, both types of membranes are suitable for electrodriven separation of mono-, bi- and tri-valent electrolytes.     

Impact of electrodialytic parameters on cation migration kinetics and fouling nature of ion-exchange membranes during treatment of solutions with different magnesium/calcium ratios

During electrodialysis (ED) treatment of solutions with different Mg/Ca ratios (R = 0, 1/20, 1/10, 1/5 and 2/5) and in different pH conditions (acid, neutral and basic), foulings on ion-exchange membranes were previously characterized and identified, by the way of X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. A mineral fouling was observed in neutral and basic conditions (for R = 1/5 and 2/5) on the anion-exchange membrane (AEM) concentrate side and in basic conditions on the cation-exchange membrane (CEM) concentrate side as well as on the diluate side for R = 1/5 and 2/5. The objectives of this present work were to link the morphological characterization and identification of membrane fouling to electrodialytic parameters and cation migration kinetics. It appeared that the CEM permselectivity was severely affected in basic conditions for R ≥ 1/5. The consequence of this alteration was the migration of OH⁻ through the CEM, a pH increase in the diluate compartment and different treatment durations. The calcite observed on AEM concentrate side for Mg/Ca ≥ 1/5 would be due first to the particular operating conditions such as the recirculation of the concentrate solution, and also to the supersaturated conditions reached or not at the AEM interface and favourable pH conditions.     

Potential application of anion-exchange membrane for hydrazine fuel cell electrolyte

A platinum electrocatalyst layer was directly bound to a perfluorinated anion-exchange membrane (AEM) by the electroless plating method and used for the characterization of AEM as a polymer electrolyte membrane for a direct hydrazine fuel cell. The crossover amount of hydrazine through AEM was much lower than that through the cation-exchange membrane (CEM) that did not depend on the applied current density. The fuel cell performance was far superior when using AEM than when using CEM.     

Membrane purification of Cl2 gas: I. Permeabilities as a function of temperature for Cl2, O2, N2, H2 in two types of PDMS membranes

Permeability (P) of Cl₂, O₂, N₂ and H₂ was measured in polydimethylsiloxane (PDMS) composite membranes with two different degrees of cross-linking. The permeability was measured in the low pressure range (1–3 bar absolute) over a fairly large temperature range 35–120°C. The functionalities of the membranes were compared on the basis of permeation rate and ability to separate the gases Cl₂–O₂. These results are part of an extensive survey where perfluorinated and carbon membranes are also included (not reported here). The purpose of the project is to develop an industrial membrane with high permselectivity for either O₂ or Cl₂ (depending on the type of membrane) at temperatures preferably above 70°C. Process conditions are set in an industrial project. The PDMS membranes are good candidates for this separation, having a high permeation rate for Cl₂ and a selectivity of Cl₂/O₂ in the range of 8–25 depending on temperature. Durability of the PDMS membranes in this aggressive environment is found to be very dependent on process conditions and on how the material is polymerized and cured. For documentation of durability, various silicones were tested; these results are to be reported separately.     

The effects of spinneret dimension and hollow fiber dimension on gas separation performance of ultra-thin defect-free Torlon hollow fiber membranes

A defect-free as-spun hollow fiber membrane with an ultra-thin dense-selective layer is the most desirable configuration in gas separation because it may potentially eliminate post-treatments such as silicone rubber costing, simplify membrane manufacture, and reduce production costs. However, the formation of defect-free as-spun hollow fiber membranes with an ultra-thin dense-selective layer is an extremely challenging task because of the complexity of phase inversion process during the hollow fiber fabrication and the trade-off between the formation of an ultra-thin dense-selective layer and the generation of defects. We have for the first time successfully produced defect-free as-spun Torlon^® hollow fiber membranes with an ultra-thin dense layer of around 540 Å from only a one polymer/one solvent binary system at reasonable take-up speeds of 10–50 m/min. The best O₂/N₂ permselectivity achieved is much higher than the intrinsic value of Torlon^® dense films. This is also a pioneering work systematically studying the effects of spinneret dimension and hollow fiber dimension on gas separation performance. Several interesting and important phenomena have been discovered and never been reported: (1) as the spinneret dimension increases, a higher elongation draw ratio is required to produce defect-free hollow fiber membranes; (2) the bigger the spinneret dimension, the higher the selectivity; (3) the bigger the spinneret dimension, the thinner the dense-selective layer. Mechanisms to explain the above observation have been elaborated. The keys to produce hollow fiber with enhanced permselectivity are to (1) remove die swell effects, (2) achieve finer monodisperse interstitial chain space at the dense-selective layer by an optimal draw ratio, and (3) control membrane formation by varying spinneret dimension.     

Novel CO2 selectively permeating membranes containing PETEDA dendrimer

Pentaerythrityl tetraethylenediamine (PETEDA) dendrimer was synthesized from pentaerythrityl tetrabromide and ethylenediamine. Its molecular structure was characterized by elemental analysis, Fourier transform infrared resonance (FT-IR) and hydrogen nuclear magnetic resonance (¹H NMR) spectroscopy. The composite membranes for selectively permeating CO₂ were prepared by using PETEDA-PVA blend polymer as the active layer and polyethersulfone (PES) ultrafiltration membrane as the support layer and their permselectivity was tested by pure CO₂ and CH₄ gases and the gas mixture containing 10 vol.% CO₂ and 90 vol.% CH₄, respectively. For pure gases, the membrane containing 78.6 wt% PETEDA and 21.4 wt% PVA in the blend has a CO₂ permeance of 8.14 × 10⁻⁵ cm³ (STP) cm⁻² s⁻¹ cmHg⁻¹ and CO₂/CH₄ selectivity of 52 at 143.5 cmHg feed gas pressure. While feed gas pressure is 991.2 cmHg, CO₂ permeance reaches 3.56 × 10⁻⁵ cm³ (STP) cm⁻² s⁻¹ cmHg⁻¹ and CO₂/CH₄ selectivity is 19. For the gas mixture, the membrane has a CO₂ permeance of 6.94 × 10⁻⁵ cm³ (STP) cm⁻² s⁻¹ cmHg⁻¹ with a CO₂/CH₄ selectivity of 33 at 188.5 cmHg feed gas pressure, and a CO₂ permeance of 3.29 × 10⁻⁵ cm³ (STP) cm⁻² s⁻¹ cmHg⁻¹ with a CO₂/CH₄ selectivity of 7.5 at a higher feed gas pressure of 1164 cmHg. A possible gas transport mechanism in the composite membranes is proposed by investigating the permeating behavior of pure gases and the gas mixture and analyzing possible reactions between CO₂/CH₄ gases and the PETEDA-PVA blend polymer. The effect of PETEDA content in the blend polymer on permselectivity of the composite membranes was investigated, presenting that CO₂ permeance and CO₂/CH₄ selectivity increase and CH₄ permeance decreases, respectively with PETEDA content. This is explained by that with increasing PETEDA content, the carrier content increases, and the crystallinity and free volume of the PETEDA-PVA blend decrease that were confirmed by the experimental results of X-ray diffraction spectra (XRD) and positron annihilation lifetime spectroscopy (PALS).     

Development of urethane acrylate composite ion-exchange membranes and their electrochemical characterization

With the objective of introducing antifouling characteristics into interpolymer types of cation and anion exchange membranes, the surface of these membranes was coated with a 12-microm-thick urethane acrylate layer and was cured by UV radiation of wavelengths 308 and 172 nm under a complete inert atmosphere. Different urethane acrylate composite ion exchange membranes developed were characterized in NaCl solution by measuring their ion-exchange capacity, volume fraction of water, contact angle with water, membrane conductance, and membrane potential. It was found that the electrochemical transport properties of urethane acrylate composite cation-exchange membranes were increased due to resonance stabilization of the urethane group, which acts as a weak acid and dissociates as a negatively charged urethane ion and a positively charged proton. This contributes toward the net charge density of the membrane matrix responsible for enhanced selectivity and conductivity, while for urethane acrylate composite anion-exchange membranes reduction in net charge density was responsible for reduction in electrochemical transport properties. Counterion transport number, permselectivity, and counterion diffusion coefficient values for these membranes were also estimated. Experiments were also carried out in higher homologs of sodium carboxylate solutions in order to observe the fouling tendencies of these membranes. It was concluded that it is possible to obtain antifouling characteristics of ion-exchange membranes by coating and curing thin hydrophilic layers of urethane acrylate on their surfaces without sacrificing their electrochemical transport properties.     

Supported carbon molecular sieve membranes based on a phenolic resin

The preparation of a composite carbon membrane for separation of gas mixtures is described. The membrane is formed by a thin microporous carbon layer (thickness, 2 μm) obtained by pyrolysis of a phenolic resin film supported over a macroporous carbon substrate (pore size, 1 μm; porosity, 30%). The microporous carbon layer exhibits molecular sieving properties and it allows the separation of gases depending on their molecular size. The micropore size was estimated to be around 4.2 Å. Single and mixed gas permeation experiments were performed at different temperatures between 25°C and 150°C, and pressures between 1 and 3.5 bar. The carbon membrane shows high selectivities for the separation of permanent gases like O₂/N₂ system (selectivity≈10 at 25°C). Gas mixtures like CO₂/N₂ and CO₂/CH₄ are successfully separated by means of prepared membranes. For example, the membrane prepared by carbonization at 700°C shows at 25°C the following separation factors: CO₂/N₂≈45 and CO₂/CH₄≈160.     

SPES/PWA/SiO_2复合质子交换膜的性能

以磺化聚醚砜(SPES)为基体,以不同比例的SiO2溶胶与磷钨酸(PWA)为掺杂物,制备了一种有望用于直接甲醇燃料电池(DMFC)的新型SPES/PWA/SiO2有机-无机复合膜,并经热失重分析(TGA)、差示扫描量热仪(DSC)、扫描电镜(SEM)-X射线能谱分析(EDX)等对膜的结构和性能进行了表征,探讨了复合膜用作质子交换膜的可能性.结果表明:复合膜较纯SPES膜具有更高的热稳定性、玻璃化转变温度和吸水率;虽然在室温和电池操作温度(80℃)下,复合膜的拉伸强度均低于纯SPES膜,但即使当SiO2含量高达20%(w)时,复合膜的拉伸强度仍高于Nafion112膜的;SEM图片显示SiO2和PWA在膜中分布均匀,这将有利于连续质子传输通道的形成.对于SiO2含量为15%(w),PWA含量为6%(w)的复合膜,其室温质子传导率达到了0.034S·cm-1,与Nafion112膜的相当,但其甲醇渗透率明显降低,仅为商用Nafion112膜的七分之一左右,这表明该复合膜在直接甲醇燃料电池中具有良好的应用前景.     

Reformulating the permselectivity-conductivity tradeoff relation in ion-exchange membranes

Polymer membranes used in separation applications exhibit a tradeoff between permeability and selectivity. That is, membranes that are highly permeable tend to have low selectivity and vice versa. For ion-exchange membranes used in applications such as electrodialysis and reverse electrodialysis, this tradeoff is expressed in terms of membrane permselectivity (i.e., ability to selectively permeate counter-ions over co-ions) and ionic conductivity (i.e., ability to transport ions in the presence of an electric field). The use of membrane permselectivity and ionic conductivity to illustrate a tradeoff between counter-ion throughput and counter-ion/co-ion selectivity in ion-exchange membranes complicates the analysis since permselectivity depends on the properties of the external solution and ionic conductivity depends on the transport of all mobile ions within a membrane. Furthermore, the use of these parameters restricts the analysis to ion-exchange membranes used in applications in which counter-ion/co-ion selectivity is required. In this study, the permselectivity-conductivity tradeoff relation for ion-exchange membranes is reformulated in terms of ion concentrations and diffusion coefficients in the membrane. The reformulated framework enables a direct comparison between counter-ion throughput and counter-ion/co-ion selectivity and is general. The generalizability of the reformulated tradeoff relation is demonstrated for cation-exchange membranes used in vanadium redox flow batteries.     

含叔丁基聚酰亚胺均质膜的气体分离性能

采用高温“一步法”缩聚合成了一系列含叔丁基的可溶性芳香聚酰亚胺树脂, 然后通过溶液浇注法制得相应均质薄膜, 并对其气体分离性能进行了测试, 同时研究了二酐结构和温度对聚酰亚胺均质膜气体分离性能的影响. 结果表明, 对于H₂, N₂, O₂, CO₂和CH₄ 等5种气体, 含叔丁基聚酰亚胺均质膜不仅表现出良好的透气性, 而且具有较高的气体透过选择性, 4,4'-(六氟异丙烯)二酞酸酐(6FDA)和均苯四甲酸二酐(PMDA)两类聚酰亚胺均质膜的气体分离性能最佳. 除CO₂外, 这两类聚酰亚胺均质膜的气体渗透系数随温度升高而升高, 而所有测试气体在这两种均质膜中的扩散系数和溶解度系数均随温度升高而增大.