Transport properties of thermally responsive anion exchange membranes containing N‐isopropylacrylamide in electrodialysis

Anion exchange membranes containing N‐isopropylacrylamide as a component were prepared, and their electrochemical properties were examined. The membranes were crosslinked with ethylene glycol dimethacrylate and contained weakly basic or strongly basic anion exchange groups. The dependence of electrochemical properties of the membranes (electrical resistance, transport number of anions, water content, and reduced osmotic flux) on temperature was completely different from those of the anion exchange membrane without N‐isopropylacrylamide. For example, the reduced osmotic flux decreased with increasing temperature until 40°C, and the transport number of chloride ions increased with increasing temperature from 25.0°C, although those of the conventional membrane monotonously increased or decreased. The transport numbers of various anions relative to chloride ions in electrodialysis were evaluated at a different temperature. Although the transport numbers between anions did not change appreciably in the conventional membrane with temperature, those of the anion exchange membranes with N‐isopropylacrylamide changed with a temperature dependent on the hydration degree of anions: permeation of less‐hydrated anions such as nitrate and bromide ions compared with chloride ions increased with increasing temperature, and that of strongly hydrated anions such as sulfate and fluoride ions decreased with increasing temperature. This is based on the increase or decrease in uptake of the anions in the membrane with the change in temperature because hydrophilicity of the membranes changes with temperature due to the apparent aggregation of isopropyl groups in the membranes. And the change in electrochemical properties and transport numbers of various anions relative to chloride ions with temperature was completely reversible with increasing or decreasing temperature. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 793–804, 1999     

Advance of click chemistry in anion exchange membranes for energy application

Click chemistry has attracted tremendous attention in polymer synthesis due to its high efficiency, considerable yield, and simple synthesis/work-up procedures. Among the various functional polymer materials prepared by click chemistry, anion exchange membrane (AEM) is a kind of polyelectrolyte which contains cations attached to the polymer skeleton. Click chemistry not only provides facile pathways for the preparation of AEMs but also generates diverse architectures of AEMs with robust performance. The commonly used click chemistry in AEMs consists of: (i) Diels-Alder reaction, (ii) thiol-ene, and (iii) Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC). This review will focus on the advance of click chemistry in the preparation of AEMs, especially synthetic approaches for different AEMs and their corresponding application in energy-related fields, such as fuel cells, redox flow battery, electrodialysis, and so on.     

Photophysical Properties and Photochemical Behaviour of Ruthenium(II) complexes containing the 2,2′-bipyridine and 4,4′-diphenyl-2,2′-bipyridine ligands

The temperature dependence of the emission lifetime of the series of complexes Ru(bpy)_n(4,4′-dpb) (bpy = 2,2′bipyridine, 4,4′-dpb = 4,4′-diphenyl-2,2′-bipyridine) has been studied in propionitrile/butyronitrile (4:5 v/v) solutions in the range 90–293 K. The obtained photophysical parameters show that the energy separation between the metal-to-ligand charge tranfer (³MLCT) emitting level and the photoreactive metal-centered (³MC) level changes across the series (ΔE = 3960, 4100, 4300, and 4700 cm^?1 for Ru(bpy)), Ru(bpy)2(4,4′-dpb)²⁺, Ru(bpy)(4,4′-dpb), and Ru(4,4′-dpb), respectively, where ΔE is the energy separation between the minimum of the ³MLCT potential curve and ³MLCT – ³MC crossing point. Comparison between spectral and electrochemical data indicated that the changes in ΔE are due to stabilization of the MLCT levels in complexes containing 4,4′-dpb with respect to Ru(bpy)²⁺₃. The photochemical data for the same complexes (as I^? salts) have been obtained in CH₂Cl₂ in the presence of 0.01M Cl^? upon irradiation at 462 nm. The complexes containing 4,4′-dpb are more photostable than Ru(bpy). Comparison between the data for thermal population of the ³MC photoreactive state and those for photochemistry indicated that the overall photochemical process is governed by (i) a thermal redistribution between the emitting and photoreactive excited states, and (ii) mechanistic factors, likely related to the size of the detaching ligand.     

Highly alkaline stable anion exchange membranes from nonplanar polybenzimidazole with steric hindrance backbone

The anion exchange membranes (AEMs) with both high ionic conductivity and alkali stability are always the research focus of the AEM fuel cells. Here, a novel nonplanar polymer for AEMs manufacture, mPBI‐TP‐x‐R, with excellent hydroxide stability and satisfactory processability is reported for the first time. The serial mPBI‐TP‐x resins with steric hindrance were prepared by copolymerization among 3,3′,4,4′‐tetraaminobiphenyl, isophthalic acid and tetraphenyl‐terephthalic acid (TP) in different ratios under microwave condensation. The copolymers mPBI‐TP‐x were quaternized at N1/N3‐sites of benzimidazole unit in backbone with alkyl groups (R?CH₃, C₂H₅, n‐C₃H₇, or n‐C₄H₉) to prepare soluble ionomers, and the corresponding membranes in hydroxyl ion form were prepared by a solution casting method and subsequent ion‐exchange process. The chemical structure of all membranes was characterized using FTIR and ¹H NMR spectroscopy. The properties of ion exchange capacity, water uptake, swelling ratio, tensile strength, ionic conductivity, and alkaline stability were measured. Among the prepared membranes, the mPBI‐TP‐15%‐(n‐Bu) exhibited the excellent alkaline stability (only degradation ca. 5% under 1M NaOH aqueous solution at 60 °C for 800 h) and satisfactory OH^? conductivity (46.66 mS/cm at 80 °C). The current research provides a useful exploration to commercial application of alkaline fuel cell. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 1087–1096     

Anion exchange membrane with viologen moiety as anion exchange groups and generation of photo-induced electrical potential from the membrane

An anion exchange membrane with a viologen moiety was prepared by the reaction of a film of chloromethylated polysulfone and 4,4′-bipyridine. The anion exchange membrane showed a high electrical resistance and a high transport number of anions due to the development of high crosslinking by the diamine. After the membrane had been swollen with ethylene glycol, photo-voltage and photo-current (82 mV, 410 nA at 200 kΩ load, 160 μm thick membrane) were generated from the membrane upon photo-irradiation.     

Improving the conductivity and dimensional stability of anion exchange membranes by grafting of quaternized dendrons

High conductivity is critical for the practical applications of anion exchange membranes (AEMs) in fuel cells. In this study, a new strategy for enhanced conductivity and dimensional stability of AEMs by incorporating quaternized dendrons is proposed. Thanks to the introduced quaternized dendrons, distinct nanoscale phase separation and well-connected ion conductive channels are formed in the as-prepared membranes (PPO-QG-x). As a result, PPO-QG-x AEMs achieve high hydroxide conductivities up to 65.5 mS cm⁻¹ at 20 °C and 121.5 mS cm⁻¹ at 80 °C (IEC = 1.95 mmol g⁻¹), while possessing good dimensional stability. Meanwhile, PPO-QG-x AEMs show good alkaline stability with the maximum loss in conductivity of 15.1% after treated in 2 M NaOH at 80 °C for 960 h. In addition, the single-cell assembled with PPO-QG-12 membrane exhibit a peak power density of 249.4 mW cm⁻² at 60 °C. Overall, this work provides a new insight to achieve high conductivity of AEMs.     

Comb‐shaped guanidinium functionalized poly(ether sulfone)s for anion exchange membranes: Effects of the spacer types and lengths

A series of poly(ether sulfone)‐based anion exchange membranes (AEMs), tethering with guanidinium side chains with different spacers, were synthesized via azide‐alkyne cycloaddition, deprotection, and the subsequent ion exchange reactions. The designed polymer structures were verified by the ¹H NMR spectra. Because of the appropriate water uptake and formation of interconnected ionic clusters, the GPES‐3C with propyl spacer showed higher conductivity than the GPES‐1C and GPES‐9C, with methylene and nonyl spacers, respectively. Comparatively, the GPES‐EO AEM with two ethylene oxide (EO) spacers exhibited even higher conductivity, these can be interpreted by interconnectivity of ionic channels and hydrophilicity nature of the EO spacer. Additionally, although the GPES membranes displayed sufficient thermal stability, the chemical stability of as‐prepared materials needs to be much improved for fuel cell applications. Overall, these results demonstrated that the properties of “pendent‐type” AEM can be tuned facilely by the spacer types and lengths. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 1313–1321     

Transport properties of anion exchange membranes prepared by the reaction of crosslinked membranes having chloromethyl groups with 4‐vinylpyridine and trimethylamine

The electrodialytic transport properties of new anion exchange membranes were evaluated that included the transport numbers of various anions, sulfate, bromide, fluoride, and nitrate ions, relative to chloride ions and current efficiency. The anion exchange membranes were prepared by the reaction of copolymer membranes crosslinked to different extents having chloromethyl groups with 4‐vinylpyridine to form a ladder‐like polymer in the membranes and then with trimethylamine to convert the remaining chloromethyl groups to benzyl trimethylammonium groups. The transport numbers of the sulfate and fluoride ions relative to the chloride ions were markedly less for the membranes that had been reacted with 4‐vinylpyridine and then with trimethylamine compared with those of the membranes that had been reacted only with trimethylamine. On the other hand, the selective permeation of nitrate and bromide ions through the membranes was enhanced by the reaction with 4‐vinylpyridine although the membranes became tighter by the reaction. The decrease in permeation of the sulfate ions was attributed to a synergistic effect involving the decrease in sulfate ions ion‐exchanged with the membranes and the decrease in mobility of the sulfate ions in the membranes with a low degree of crosslinking. Though the ion‐exchanged sulfate ion content was the lowest in the highly crosslinked membranes, the mobility ratio between the sulfate ions and chloride ions did not decrease in the membranes. However, the increase in the permeation of nitrate ions was based on the increase in the ion‐exchanged amount of nitrate ions with the membrane, and not the change in the mobility ratio between the nitrate and chloride ions. The formation of the ladder‐like polymer in the membrane matrix brought on a decrease in the hydrophilicity of the membranes due to pyridine groups and an increase in their tightness. The current efficiency of all membranes was greater than 99% during the electrodialysis of 0.50 N salt solutions. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1773–1785, 1999     

Preparation and characterization of Type II anion exchange membranes from poly(2,6-dimethyl-1,4-phenylene oxide) (PPO)

To separate hydrophilic anions from hydrophobic ones, Type II PPO-based anion exchange membranes were developed. Different from Type I (with both trimethylbenzylammonium and triethylbenzylammonium groups), Type II has an excellent hydrophobicity modifier as fixed groups: dimethyethanolammonium groups, which were introduced into PPO (poly(2,6-dimethyl-1,4-phenylene oxide)) by following benzyl bromination of PPO and subsequent quaternary amination with a dimethylethanolamine (DMEA) aqueous solution. The membrane's intrinsic properties are dependent on DMEA concentration and amination temperature. The optimum conditions for membrane preparation are as follows: amination temperature 70 °C, time 30–48 h, and DMEA concentration 1:3–1:5 (v/v, DMEA to water). The obtained Type II anion exchange membranes had an IEC of 1.5 mmol/g dry membrane, water content of 30%, and membrane area resistance of 30 Ω cm². The introduced dimethyethanolammonium groups can block hydrated anions from the access to membranes but let hydrophobic anions transport; hence, an effective separation between hydrophilic and hydrophobic anions can be achieved during electro-membrane operation.     

Synthesis of guanidinium‐based anion exchange membranes and their stability assessment

Alkaline fuel cells potentially offer improved conversion efficiency and the prospect of using non‐noble metal catalysts; however, low conductivity and fast degradation of anion exchange membranes (AEMs) prevent their widespread application. In this work, a series of novel composite AEMs were synthesized by incorporating guanidinium‐based polymers into a porous polytetrafluoroethylene (PTFE) film. The guanidinium‐based polymers were polymerized using a condensation process between a guanidinium salt and two different diamines so that the guanidinium cations were tethered to the polymer backbone to enhance both conductivity and durability. In addition, polymer crosslinking was conducted to further reinforce the mechanical strength of the membranes and interlock the guanidinium moieties to the porous PTFE. It was found that the ionic conductivity of the synthesized membrane reached up to approximately 80 mS cm^?1 at 20°C in deionized water. These membranes also exhibited superior stability compared to commercial quaternary ammonium AEMs after being exposed in 5 M KOH solution at 55°C for 50 h. Copyright © 2013 John Wiley & Sons, Ltd.     

Preparation of alkaline anion exchange membranes based on functional poly(ether-imide) polymers for potential fuel cell applications

A novel poly(ether-imide)-based alkaline anion exchange membrane with no free base has been prepared and characterized for its ionic conductivity in water, which is a critical metric of its applicability in a liquid-fed direct methanol fuel cell. The poly(ether-imide)-based membranes were prepared by chloromethylation, quaternization and alkalization of commercial poly(ether-imide) and the derivatives were characterized by NMR. The chemical and thermal stabilities were investigated by measuring changes of ionic conductivities when the membranes were placed in various alkaline concentrations and temperatures for 24 h. The membranes were stable at all concentrations of KOH at room temperature, but not at elevated temperatures. The membranes were stable in 1.0 M KOH solution up to 80 °C without losing membrane integrity. The measured conductivity of the formed membrane ranged from 2.28 to 3.51 × 10⁻³ S/cm at room temperature. This preliminary study indicates that functionalized poly(ether-imide) has suitable conductivity suggesting that it can be used as an alkaline anion exchange membrane in fuel cell applications.     

Formation of pore‐filled ion‐exchange membranes with in situ crosslinking: Poly(vinylbenzyl ammonium salt)‐filled membranes

Robust, polyelectrolyte‐filled, microporous membranes were prepared by the introduction and crosslinking of a preformed polymer within the pores of a poly(propylene) host membrane. Specifically, poly(vinylbenzyl chloride) (PVBCl) was reacted with piperazine or 1,4‐diaminobicyclo[2.2.2]octane in an N,N‐dimethylformamide (DMF) solution contained in the pores of the microporous base membrane. The remaining chloromethyl groups were reacted with an amine, such as trimethylamine, to form positively charged ammonium sites. This simple two‐step procedure gave dimensionally stable, anion‐exchange membranes in which the degree of crosslinking and the mass loading were determined by the concentration of PVBCl and crosslinker in the starting DMF solution. The incorporated polyelectrolyte gel was evenly distributed within the pores of the host membrane with no surface layers present. The membranes are fully characterized. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 807–820, 2001     

Effects of temperature and anion type on the two-dimensional condensation of the 4,4′-bipyridine cation radical on mercury

The reduction of 4,4′-bipyridine (BPH²⁺₂) on mercury in an acid medium gives a very narrow sharp tail-less reversible voltammetric peak that can be ascribed to the formation of a two-dimensional (2D) phase of the cation radical BPH^·+₂ at the electrode according to the reaction

BPH²⁺₂ + e⁻ |BPH^·+₂|_2D

The corresponding oxidation peak possesses similar properties and arises from the destruction (fusion) of the 2D phase.In this work we studied the influence of some experimental variables, namely the type of anion present in the medium, the concentration of 4,4′-bipyridine and temperature on the 2D phase transition peaks. Also, we tested various analytical criteria to validate this assignation and fitted both voltammetric peaks numerically to the theoretical model developed for this purpose.     

Synthesis and structure–property relationships of poly(sulfone)s for anion exchange membranes

Membranes based on cationic polymers that conduct anions are important for enabling alkaline membrane fuel cells and other solid-state electrochemical devices that operate at high pH. Anion exchange membranes with poly(arylene ether sulfone) backbones are demonstrated by two routes: chloromethylation of commercially available poly(sulfone)s or radical bromination of benzylmethyl moieties in poly(sulfone)s containing tetramethylbisphenol A monomer residues. Polymers with tethered trimethylbenzyl ammonium moieties resulted from conversion of the halomethyl groups by quaternization with trimethyl amine. The water uptake of the chloromethylated polymers was dependent on the type of poly(sulfone) backbone for a given IEC. Bisphenol A-based Udel® poly(sulfone) membranes swelled in water to a large extent while membranes from biphenol-based Radel® poly(sulfone), a stiffer backbone than Udel, only showed moderate water uptake. The water uptake of cationic poly(sulfone)s was further reduced by synthesizing tetramethylbisphenol A and 4,4′-biphenol-containing poly(sulfone) copolymers where the ionic groups were clustered on the tetramethylbisphenol A residues. The conductivity of all samples scaled with the bulk water uptake. The hydration number of the membranes could be increased by casting membranes from the ionic form polymers versus converting the halomethyl form cast polymers to ionic form in the solid state. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1790–1798, 2013     

Diastereoselective Ion Pairing of TRISPHAT Anions and Tris(4,4′-dimethyl-2,2′-bipyridine)iron(II)

Two configurationally stable, chiral anions (TRISPHAT, 1 ) behave as efficient hosts that control the configuration of a configurationally labile iron(II ) complex as the guest with high diastereoselectivity (>96 % de) upon ion pairing. The diastereoselectivity increases with decreasing solvent polarity.     

Effect of crosslinking on the properties of partially fluorinated anion exchange membranes

A series of crosslinked, ammonium‐functionalized, and partially fluorinated copolymers have been prepared and evaluated as anion exchange membranes. In order to investigate the effect of crosslinking on the membrane properties, precursor copolymers containing chloromethyl groups were crosslinked with various aliphatic diamines followed by quaternization with monoamines. Crosslinking was effective in lowering water absorbability at no expense of high hydroxide ion conductivity of the membranes. By tuning the degree of crosslinking (20 mol %) and crosslinker chain length (C6 and C8), the highest ion conductivity of 73 mS/cm (at 80°C in water) was achieved. Furthermore, alkaline stability of the membranes was also improved by the crosslinking; the remaining ion conductivity after the stability test (in 1 M potassium hydroxide at 80°C) was 8.2 mS/cm (after 1000 h) for the C6 crosslinked membrane and 1 mS/cm (after 500 h) for the uncrosslinked membrane, respectively. The ammonium groups attached with the crosslinkers seemed more alkaline stable than the uncrosslinked benzyltrimethylammonium groups, while the polymer main chain was intact under the harsh alkaline conditions. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 1059–1069     

Cation exchange membranes by pre-irradiation grafting of styrene into FEP films. II. Properties of copolymer membranes

The structure and some physico-chemical properties of radiation grafted FEP-g-polystyrenesulfonic acid proton exchange membranes were studied as a function of the degree of grafting. The distribution of grafted polymer across the membrane thickness was obtained from microprobe measurements. It was found that for low levels of grafting (ca. 3%), polystyrene chains are located near the membrane surface only, and the interior of the membrane remains ungrafted. With the increasing degree of grafting, polystyrene chains were incorporated into the interior of the membrane as well. An almost homogeneous distribution of grafts in the membrane was obtained at a graft level of > 13%. The influence of the degree of grafting on membrane properties, such as ion exchange capacity, swelling, and specific resistivity was studied. Three different states of water, viz., freezing free, freezing bound, and nonfreezing water have been identified in noncrosslinked membranes. However, the nature and the amount of crosslinker had a profound influence on the states of water in a membrane. © 1996 John Wiley & Sons, Inc.     

Optimization of anionic conductivity through the coexistence of ionomer cluster and backbone-backbone morphologies in anion exchange membranes

Random copolymers of poly(4-vinylpyridine) and polyisoprene were synthesized, and subsequently quaternized with 1-alkylbromides. The number of carbons on the pendant side-chain of the resultant comb-shaped polymer, n, ranged from 2–8. The comb-shaped polymers were crosslinked employing thiol-ene chemistry to give mechanically robust ion conducting membranes. Analysis by wide and medium-angle X-ray scattering show three morphology regimes that are dependent on the number of carbons on the pendant side-chains. When n = 2, ionomer cluster morphology was dominant, when n = 8 backbone-backbone morphology was dominant, and when n = 3–6, the membrane showed a coexistence of both ionomer cluster and backbone-backbone morphologies. Evaluation of the water uptake of the membranes showed a maximum water uptake per cation of 9.5 when n = 5 at 95% relative humidity (RH) and 60°C. Conductivity of the samples characterized by electrochemical impedance spectroscopy showed bromide conductivity as high as 110 mS/cm when n = 3 at 95% RH and 90°C.     

Effect of ammonium groups on the properties and alkaline stability of poly(arylene ether)‐based anion exchange membranes

Five kinds of ammonium groups functionalized partially fluorinated poly(arylene ether) block copolymer membranes were prepared for investigating the structure–property relationship as anion exchange membranes (AEMs). Consequently, the pyridine (PYR)‐modified membrane showed the highest alkaline and hydrazine stability in terms of the conductivity, water uptake, and dry weight. The chloromethylated precursor block copolymers were reacted with amines, such as trimethylamine, N‐butyldimethylamine, 1‐methylimidazole, 1,2‐dimethylimidazole, and PYR to provide the target quaternized poly(arylene ether)s. The structures of the polymers, as well as model compounds and oligomers were well characterized by ¹H NMR spectra. The obtained AEMs were subjected to water uptake and hydroxide ion conductivity measurements and stabilities in aqueous alkaline and hydrazine media. The pyridinium‐functionalized quaternized polymers membrane showed the highest alkaline and hydrazine stability with minor losses in the conductivity, water uptake, and dry weight. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 383–389     

Synthesis and characterisation of sulfonic acid-containing ion exchange membranes based on hydrocarbon and fluorocarbon polymers

Radiation-induced graft copolymerisation has been used to modify polymers with styrene to prepare pre-cursor copolymers that can be subsequently functionalised to produce ion exchange membranes. This paper describes the processes of simultaneous and pre-irradiation graft copolymerisation of styrene to modify hydrocarbon and fluorine-containing polymers and their sulfonation to produce hydrophilic membranes. The effect of varying the grafting conditions and their characterisation by ion exchange capacity, electrolytic resistivity and equilibrium water content is reported.