Fundamental studies on the intermediate layer of a bipolar membrane part IV. Effect of polyvinyl alcohol (PVA) on water dissociation at the interface of a bipolar membrane

This paper investigates the effect of polyvinyl alcohol (PVA) as the intermediate layer of bipolar membranes on water dissociation. The bipolar membranes are prepared by coating a solution of sulfonated polyphenylene oxide (SPPO) on the anion exchange layers, which had been immersed in PVA aqueous solutions with different concentrations in advance. The experimental results show that the effect of PVA on water dissociation is significantly affected by its concentration: at low concentration range PVA solution catalyzes water dissociation and at high concentration range PVA solution shows a retardant effect. The phenomenon is different from the situation where polyethylene glycol (PEG) is used as the intermediate layer. Based on the adsorption data and the measurements of X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy, it is found that (1) the effect of PVA concentration on water dissociation can be explained by the competition and compromise of two effects--the catalytic and hydrophilic effect of the PVA molecule and the steric effect of the enlargement of junction thickness; (2) the reason that at high concentration range PVA and PEG show different effects on water dissociation is that the junction thickness is enlarged excessively by PVA, which is based on the intrinsic characteristics of PVA molecules.     

Fundamental studies on the intermediate layer of a bipolar membrane V. Effect of silver halide and its dope in gelatin on water dissociation at the interface of a bipolar membrane

The effect of silver ions on the water dissociation of bipolar membranes was first investigated in this paper. To do this, the bipolar membranes were prepared by immersing the anion exchange layers in an AgNO3 solution and then coating a solution of sulfonated polyphenylene oxide (SPPO) on the anion exchange layers. XPS and AES observations indicated that silver at the intermediate layer was in the form of AgCl. The experimental results proved that AgCl has an excellent catalytic function for water dissociation in terms of I-V curves, and the quantity of AgCl played an important role in the behavior of a bipolar membrane. The bipolar membranes with gelatin and the gelatin doped with silver as a catalytic layer were also prepared in the same way, and their I-V behavior and the water dissociation pilot tests were also investigated. The experimental results showed that in the case of gelatin alone, the voltage drop increased slightly at high gelatin concentrations, due mainly to the steric effect and electrostatic interaction, but decreased at low gelatin concentrations due to the hydrophilicity. However, when gelatin was doped with AgCl, the bipolar membranes have an appreciable improvement in both stability and catalytic function, in comparison with those prepared from silver or gelatin.     

Nanocomposite interface coupled with thickness optimization promoting water dissociation in heterogeneous bipolar membrane

Bipolar membranes (BPMs) are multilayered composite film containing an interface layer sandwiched between cation exchange layer (CEL) and anion exchange layer (AEL), and are capable of dissociating water molecules under reverse bias potential. Woven fabric supported heterogeneous bipolar membranes (HBMs) were synthesized adopting layer-by-layer solvent casting technique. Nanocomposite layer based on sulfonated polyether ether ketone (SPEEK) and GO (graphene oxide) were applied at the interface of CEL/AEL made of cation/anion exchange resins and poly (vinyl chloride) as binder to advance water dissociation in HBMs. Thickness of monopolar layers were initially optimized without any interfacial layer. Introduction of SPEEK interface substantially lowered onset water dissociation potential, U_diss (~1.87 V) relative to the HBM without interface (~3.27 V), which got further reduced (~1.80 V) by nanocomposite (GO + SPEEK) interface. U_diss recorded with SPEEK + GO as interface was much lower than some of the recently reported homogeneous BPM. The NaOH production from NaCl (1.0 mol?L^?1) solution in a bipolar membrane electrodialysis set up containing synthesized HBM with nanocomposite interface (SPEEK + GO) was double than that of NaOH concentration obtained with HBM having no interface, where the current density was fixed at 50.0 mA·cm^?2. Careful optimization of monopolar/interface layer thickness and composition of nanocomposite interface results in developing cost effective HBMs facilitating water dissociation at lower potential.     

Fundamental studies on the intermediate layer of a bipolar membrane: Part III. Effect of starburst dendrimer PAMAM on water dissociation at the interface of a bipolar membrane

Starburst dendrimer polyamidoamine (PAMAM) with ellipsoidal or spheroidal shape is structure-regular and has much more amino groups than conventional polymers. This paper investigates the possibility of these amino groups on water dissociation in a bipolar membrane interface. To do this, a bipolar membrane is prepared by casting the solution of sulfonated poly(phenylene oxide) (SPPO) in dimethyl formamide (DMF) on a commercial anion exchange membrane that is immersed in PAMAM aqueous solution in advance. The existence of PAMAM adsorbed on the membrane is proved by X-ray photoelectron spectroscopy (XPS), and the adsorption amount is evaluated by weighting method. The junction thickness of the prepared bipolar membrane is determined by electrochemical impedance spectroscopy (EIS), and the performance is evaluated by current–voltage curves. The experiments show that both the generation and concentration of PAMAM would strongly affect the characteristics of the bipolar membranes. There exists a transitional concentration for various generations PAMAMs to catalyze effectively the water dissociation, and above or below the transitional concentration the performance of bipolar membranes is decreasing. The higher the generation, the lower the concentration. Moreover, at a fixed solution concentration, there is not the simple relation of monotone decreasing or increasing between the performance of bipolar membranes and the generations of PAMAMs. All these can be explained according to the characteristics of PAMAMs combined with available water dissociation theory.     

mCMC-PEG/mCS-PEG双极膜的制备与表征

以Fe3+改性羧甲基纤维素(mCMC)和聚乙二醇(PEG)共混为阳膜;以戊二醛改性壳聚糖(mCS)和聚乙二醇共混为阴膜,制备了mCMC-PEG/mCS-PEG双极膜.以FTIR测定了膜红外光谱,以扫描电镜观察了膜表面和界面层的形态,以TG进行膜的热重分析.测定了mCMC-PEG和mCS-PEG不同比例共混膜的含水率、离子交换容量、溶胀度,及mCMC-PEG/mCS-PEG双极膜的电性能.研究结果表明,在双极膜材料中引入亲水性的聚乙二醇后,因分子间的相容性增大,故而提高了双极膜的离子交换容量,并减小了膜的溶胀性.当CMC∶PEG质量比等于10∶1和CS∶PEG质量比等于2∶1时所制得的双极膜具有良好的电化学性能,在酸碱溶液中机械强度高、溶胀小.     

Catalytic water dissociation using hyperbranched aliphatic polyester (Boltorn series) as the interface of a bipolar membrane

The effect of hyperbranched aliphatic polyester (Boltorn series) on the water dissociation in bipolar membranes was firstly investigated in this paper. The bipolar membranes were prepared by immersing the anion exchange layer in a hyperbranched aliphatic polyester solution and then coating on the layer a polyphenylene oxide (SPPO) solution. The SEM observations proved the existence of hyperbranched aliphatic polyester at the membrane intermediate layer. The adsorption amount was evaluated by the oxygen content via XPS. The junction thickness of the prepared bipolar membrane was determined by electrochemical impedance spectroscopy (EIS), and the membrane performances were evaluated by current-voltage curves. The results showed that the amount and generation of Boltorn series, and temperature all affected I-V behaviors of the fabricated bipolar membranes, and the former two played the critical role. These effects were explained on the basis of the water dissociation theory and the characteristics of hyperbranched aliphatic polyester.     

Fundamental studies on the intermediate layer of a bipolar membrane. Part II. Effect of bovine serum albumin (BSA) on water dissociation at the interface of a bipolar membrane

This paper investigates the behavior of bovine serum albumin (BSA) during water dissociation on a bipolar membrane (BPM). BSA-modified BPM is prepared by immersing polyethylene anion exchange membrane in different concentration solutions of BSA, then casting the solution of sulfonated poly(phenylene oxide) (SPPO) in dimethyl formamide. The modification of BSA was evidenced by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The junction thickness was evaluated by electrochemical impedance spectroscopy (EIS). The results showed that the typical I-V curves for bipolar membranes were heavily affected by the BSA modifications: the more the adsorbed amount of BSA, the larger the potential drop across a bipolar membrane. The new phenomena is underlined by the intrinsic properties of BSA molecules: steric effects give rise to an increase in the thickness of the depletion layer, amphoteric properties weaken the electric field of the junction, and hydrophobicity makes the junction less wet. All of these cause negative effects on water dissociation on a bipolar membrane.     

Limiting current density and water dissociation in bipolar membranes

The behaviour of bipolar membranes in NaCl and Na₂SO₄ solutions is discussed. The membranes are characterized in terms of their limiting current densities. Below the limiting current density the electric current is carried by salt ions migrating from the transition region between the anion and the cation exchange layer of the bipolar membrane. In steady state these ions are replaced by salt ions transported from the bulk solutions into the transition region by diffusion and migration due to the fact that the ion-exchange layers are not strictly permselective. When the limiting current density is exceeded, the salt transport from the transition region can no longer be compensated by the transport into the region and a drastic increase in the membrane resistance and enhanced water dissociation is observed. This water dissociation is described as being a combination of the second Wien effect and the protonation and deprotonation of functional groups in the membrane. The limiting current density is calculated from a mass balance that includes all components involved in the transport. The parameters used in the mathematical treatment are the diffusion coefficients of salt ions and water, the ion mobilities in the membrane, the fixed charge densitiy of the membrane, the pK_b values of the functional groups and the solution bulk concentrations.     

Study of pH correction process of chloride-bicarbonate dilute solutions by electrodialysis with bipolar membranes

The pH of a dilute chloride-hydrocarbonate solution and the concentrations of chloride ions and carbonic acid anions at the outlet of the alkaline and acid chambers of the electrodialysis cell formed by bipolar and anion-exchange membranes were determined. The decrease in the concentration of hydrocarbonate ions in the alkaline chamber with growth of current density was not equal to its increase in the acid chamber. This disbalance was caused by two concurrent processes: the electromigration ion transport through the anion-exchange membrane and the chemical reactions of hydrocarbonate ions with the water dissociation products formed on the bipolar and anion-exchange membranes. A mathematical model was suggested to describe the electrodialysis correction of the pH of a dilute chloride-hydrocarbonate solution. The experimental data on the correction of pH of the chloride-hydrocarbonate solution were well approximated by both the model that takes into account water dissociation on the anion-exchange membrane and the simplified model that neglects water dissociation. The experimental data agreed well with the results of calculations by the model in which the effective anion transport numbers were calculated only from ion concentrations and diffusion coefficients in solution. This reflects the outer diffusion character of the kinetics of ion transport through the anion-exchange membrane, with pH of dilute solutions corrected by electrodialysis.     

CuPc(COOH)8-SA/CuTAPc-CS双极膜的制备及表征

分别用八羧基铜酞菁[CuPc(COOH)₈]和四氨基铜酞菁(CuTAPc)改性海藻酸钠(SA)阳膜层和壳聚糖(CS)阴膜层, 制备了CuPc(COOH)₈-SA/CuTAPc-CS双极膜. 实验结果表明, 经八羧基铜酞菁和四氨基铜酞菁改性后, 促进了双极膜中间层水的解离, 增大了阳离子交换膜层和阴离子交换膜层的离子交换容量及H⁺和OH^-的透过率. 与Fe³⁺改性的Fe-SA/mCS双极膜相比, CuPc(COOH)₈-SA/CuTAPc-CS双极膜的阻抗、电阻压降(即IR降)和溶胀度降低. 当电流密度高达120 mA/cm²时, CuPc(COOH)₈-SA/CuTAPc-CS双极膜的IR降仅为0.9 V.     

Polymers with Tethered Anionic and Cationic Groups as Membranes for Fuel Cells

Ionic clustering, water binding, and ion conductivity were studied in polymers functionalized with sulfonic acid and quaternary ammonium hydroxide groups. Small-angle x-ray scattering showed that no clustering occurred in the quaternary ammonium containing anion exchange membranes, while evidence of ionic clusters was present in both sulfonated poly(phenylene) and in Nafion, a poly(perfluorosulfonic acid). Interestingly, the water self-diffusion coefficients of the anion exchange membranes were generally greater than those observed for the sulfonated poly- (phenylene)s, and moreover, the water self diffusion coefficients in anion exchange membranes were not a strong function of diffusion time. The water binding behavior lead to increased normalized conductivity in anion exchange membranes as compared to proton exchange membranes at the highest ion exchange capacities.     

Transport properties of electrodialysis membranes in the presence of Zn2+ complexes with Cl−

The aim of this paper is to investigate the properties of anion exchange electrodialysis membranes in contact with aqueous solutions containing zinc chloride complexes. Measurements of electric resistance, water content, quantity of sorbed electrolytes, self-diffusion and electrotransport ionic fluxes were performed as a function of the composition of the external solutions. The results were analysed in relation to the complex formation inside the solutions.     

Mass transport in a boundary layer and in an ion exchange membrane: Mechanism of concentration polarization and water dissociation

In an unforced flowing NaCl solution in bulk, gravitational or electro convection supplies ions from bulk toward the membrane surface through a boundary layer. In a boundary layer formed on an anion exchange membrane, the convection converts to migration and diffusion and carries an electric current. In a boundary layer formed on a cation exchange membrane, the convection converts to migration and carry an electric current. In a forced flowing solution in bulk, the boundary layer thickness is reduced and gravitation or electro convection is disappeared. An electric current is carried by diffusion and migration on the anion exchange membrane and by migration on the cation exchange membrane. Ion transport in a boundary layer on the cation exchange membrane immersed in a NaCl solution is more restricted comparing to the phenomenon on the anion exchange membrane. This is due to lower counter-ion mobility in the boundary layer and the restricted water dissociation reaction in the membrane. The water dissociation reaction is generated in an ion exchange membrane and promoted due to the increased forward reaction rate constant. However, the current efficiency for the water dissociation reaction is generally low. The intensity of the water dissociation is more suppressed in the strong acid cation exchange membrane comparing to the phenomenon in the strong base anion exchange membrane due to lower forward reaction rate constant in the cation exchange membrane. In the strong acid cation exchange membrane, the intensity of electric potential is larger than the values in the strong base anion exchange membrane. Accordingly, the stronger repulsive force is developed between ion exchange groups (SO ₃ ^? groups) and co-ions (OH^? ions) in the cation exchange membrane, and the water dissociation reaction is suppressed. In the strong base anion exchange membrane, the repulsive force between ion exchange groups (N⁺(CH₃)₃ groups) and co-ions (H⁺ ions) is relatively low, and the water dissociation reaction is not suppressed. Violent water dissociation is generated in metallic hydroxides precipitated on the desalting surface of the cation exchange membrane. This phenomenon is caused by a catalytic effect of metallic hydroxides. Such violent water dissociation does not occur on the anion exchange membrane.     

mPAM/mCMC双极膜电解槽中MnO2电催化氧化制备甘油醛

分别以FeCl3和戊二醛等对羧甲基纤维素(CMC)和聚丙烯酰胺(PAM)进行改性, 制备了mPAM/mCMC双极膜. 测定了PAM、CMC胶体的电荷密度, mPAM/mCMC双极膜离子交换能力、I-V工作曲线等参数. 用扫描电镜和红外光谱对膜形貌与成分作表征, 膜厚≈260 μm, 中间界面层厚为纳米级. 热重分析表明膜具有较好的热稳定性. 以mPAM/mCMC双极膜为电解槽的隔膜, 间接电氧化甘油为甘油醛. 在电场的作用下, 双极膜中间层中的水离解产生H＋和OH−, OH−及时地传输入阳极室, 中和了电生成甘油醛时生成的H＋, 促进了正向反应的进行. 槽电压稳定, 产率达91.6%, 电流效率为65.5%.     

Correction of pH of diluted solutions of electrolytes by electrodialysis with bipolar membranes

The current efficiencies of the water dissociation water and the voltage-current characteristics of the bipolar (asymmetric bipolar) membranes were measured in a two-chamber electrochemical cell. The cell was formed of an MB-3 bipolar membrane or an asymmetric bipolar membrane, which is an MA-40 heterogeneous membrane with a thin surface layer in the form of a cation-selective homogeneous film and MA-40 and MA-41 heterogeneous monopolar membranes. The dissociation of water on MA-40 in 0.01 M sodium chloride decreased the current efficiency of the acid and alkali both in the channel with a bipolar membrane and in the channel with an asymmetric bipolar membrane. The effective ion transport numbers across MA-40 and MA-41 at different pH values were determined. The water dissociation rate on MA-40 decreased at pH > 9.5. A kinetic model of the electrodialysis of a dilute solution of sodium chloride in a two-chamber unit cell with a bipolar and anionite membranes was suggested.     

Radiation-Induced Asymmetric Grafting of Different Monomers into Base Films to Prepare Novel Bipolar Membranes

We prepared novel bipolar membranes (BPMs) consisting of cation and anion exchange layers (CEL and AEL) using radiation-induced asymmetric graft polymerization (RIAGP). In this technique, graft polymers containing cation and anion exchange groups were introduced into a base film from each side. To create a clear CEL/AEL boundary, grafting reactions were performed from each surface side using two graft monomer solutions, which are immiscible in each other. Sodium p-styrenesulfonate (SSS) and acrylic acid (AA) in water were co-grafted from one side of the base ethylene-co-tetrafluoroethylene film, and chloromethyl styrene (CMS) in xylene was simultaneously grafted from the other side, and then the CMS units were quaternized to afford a BPM. The distinct SSS + AA- and CMS-grafted layers were formed owing to the immiscibility of hydrophilic SSS + AA and hydrophobic CMS monomer solutions. This is the first BPM with a clear CEL/AEL boundary prepared by RIAGP. However, in this BPM, the CEL was considerably thinner than the AEL, which may be a problem in practical applications. Then, by using different starting times of the first SSS+AA and second CMS grafting reactions, the CEL and AEL thicknesses was found to be controlled in RIAGP.     

Deacidification of citric acid solutions by electrodialysis

Electrodialysis is a useful process to deacidify citrus juices. Besides the known allanion exchange membrane process, two alternative electrodialysis processes were investigated: a three-stream process using cation and anion exchange membranes and a twostream process with alternately arranged bipolar and anion exchange membranes. The results taken from a laboratory electrodialysis cell show, that according to current efficiency, consumption of sodium hydroxide and by-production of sodium citrate or citric acid the alternative processes are favourable, especially the process using bipolar membranes.     

Assisted Electromigration of Bipolar Ions through Ion-Selective Membranes in Glycine Solutions

At current densities above the limiting diffusion currents in electromembrane systems, bipolar ions of amino acids react with protons or hydroxyl ions that form during irreversible dissociation of water molecules at membrane–solution interfaces, convert into cations or anions, and then, after a gradient of electric potential is imposed on the system, migrate through cation-selective or anion-selective membranes (assisted electromigration). This phenomenon makes it possible to effectively separate solutions containing bipolar ions and nonelectrolyte molecules using electrodialysis with alternate cation-exchange and anion-exchange membranes.     

阴离子交换膜的制备和改性研究进展

离子膜交换有广泛的应用,本文介绍了有机阴离子交换膜的修饰和改性以及新兴的有机-无机杂化阴离子交换膜的制备方法和发展现状,特别是介绍了近年来开发的一些不需要使用氯甲醚的阴离子膜制备新路线。     

Effect of the Interface Component on Current-voltage Curves of a Composite Bipolar Membrane for Water and Methanol Solutions

The current-voltage curves of a composite bipolar membrane (CBM) were experimentally measured by varying the interface component between cation- and anion-exchange membranes for water and methanol solutions. In each solution system, 0.05 mol/l LiCl was used as the electrolyte. The interface component was varied by pasting the polymers or installing the thin membranes in the intermediate region of the CBM. The measured results show that the functional groups of the polymers and thin membranes enhanced the water and methanol splitting effect. This phenomenon can be explained by the protonation-deprotonation reactions occurring between these functional groups and the water or methanol molecules in the intermediate region of the CBM. The effect of transition metal compounds existing in the intermediate region of the CBM was also experimentally examined in this study. It was found that the effect of transition metal compounds on water or methanol splitting was not obvious. Copyright 1999 Academic Press.