Basicity‐dependent properties of anion conducting membranes consisting of iminium cations for alkaline fuel cells

To design novel anion‐conducting polymer electrolyte membranes (AEMs), this paper proposes a basicity index (BI) that is defined by the ion‐exchange ratios of AEMs from the OH^? to Cl^? forms in a neutral aqueous solution as a parameter for Arrhenius basicity (dissociation constant). Using a radiation‐induced graft polymerization technique, three iminium cations are introduced into fluorinated polymer films. The BI of the iminium‐containing AEMs is less than that of a conventional ammonium‐type AEM. The conductivity and water uptake correlate positively with the BI, whereas the thermal and chemical stabilities correlate negatively with the BI. The dependence on the BI stems from the stabilization of the iminium hydroxide in proportion to the basicity of the original diaza‐compounds, resulting in a decrease in conductivity and water uptake with keeping higher thermal and chemical stabilities. Notably, ion conductivity is sufficient and water uptake is less in AEMs with a medium BI. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 503–510     

Utilizing thiol–ene chemistry for crosslinked nickel cation‐based anion exchange membranes

Metal cation‐based anion exchange membranes (AEMs) are a unique class of materials that have shown potential to be highly stable AEMs with competitive conductivities. Here, we expand upon previous work to report the synthesis of crosslinked nickel cation‐based AEMs formed using the thiol–ene reaction. These thiol–ene‐based samples were first characterized for their morphology, both with and without nickel cations, where the nickel‐containing membranes demonstrated a disordered scattering peak characteristic of ionic clusters. The samples were then characterized for their water uptake, chemical and mechanical stability, and conductivity. They showed a combination of high water content and extreme brittleness, which also resulted in fairly low conductivity. The brittleness resulted from large water swelling as well as the need for each nickel cation to act as a crosslinker, necessary with the current nickel‐coordination chemistry. Therefore, increasing the ion exchange capacity (IEC) for these types of AEMs, important for enhancing conductivity, also increased the crosslink density. The low conductivity and brittleness seen in this work demonstrated the need to develop non‐crosslinking metal‐complexes. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 328–339     

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     

Novel Crosslinked Alkaline Exchange Membranes Based on Poly(phthalazinone ether ketone) for Anion Exchange Membrane Fuel Cell Applications

Novel crosslinked anion exchange membranes based on poly(phthalazinone ether ketone) (PPEK) were successfully prepared through chloromethylation, quaternization, membrane casting and OH‐ ionic exchange reaction from the quaternized PPEK (QPPEK) membrane. The quaternization was performed with N‐methylimidazolium (MIm) as ammonium agent and tetramethylethylenediamine (TMEDA) as crosslinking agent. The ion‐exchange capacity, swelling ratio (SR), water uptake (WU), and ionic conductivity of the QPPEK alkaline membranes have been systematically investigated. The results showed that QPPEK membranes have a high hydroxide conductivity and very low SR. For the QPPEK‐4 alkaline membrane with ion‐exchange capacity (IEC) 2.63 mmol/g, the WU was 35.8%, and the hydroxide conductivity was 0.028 S/cm at 30 °C and 0.032 S/cm at 70 °C, while its SR was only 7.6%. The thermal properties of the QPPEK alkaline membrane and CMPPEK were characterized using thermo‐gravimetric analysis measurements in a nitrogen atmosphere. The alkaline resistance of membrane QPPEK ?4 was also briefly investigated in 6 M KOH at 60 °C. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1632–1638     

Preparation and characterization of anion‐exchange membranes derived from poly(vinylbenzyl chloride‐co‐styrene) and intercalated montmorillonite

In this paper, three organic intercalating agents containing cations [hexadecyl trimethyl ammonium bromide (CTAB), poly(acrylamide‐co‐diallyldimethylammonium chloride), and quaternized polyethyleneimine] are used to prepare intercalated montmorillonites (MMT) by ion‐exchange method. Then the modified MMTs are doped with vinylbenzyl chloride and styrene copolymer [poly(vinylbenzyl chloride‐co‐styrene)] for fabricating composite anion‐exchange membranes (AEM). Fourier transform infrared, X‐raydiffraction, thermogravimetric analysis, scanning electron microscopy, and Mastersizer laser particle size analyzer are employed to characterize the structure and morphology of MMTs and AEMs. The successful intercalation of MMTs is approved, and the MMT intercalated by CTAB shows an interlayer distance of 2.31 nm. The properties of the composite membranes including water uptake, mechanical property, and ionic conductivity are investigated. Among all the AEMs, the composite membrane containing MMT sheets with CTAB demonstrates better compositive performances. It presents an ionic conductivity of 2.09 × 10^?2 S cm^?1 at 80°C and good alkaline solution stability. Copyright © 2016 John Wiley & Sons, Ltd.     

Anion exchange membranes: Current status and moving forward

This short review is meant to provide the reader with highlights in anion exchange membrane research, describe current needs in the field, and point out promising directions for future work. Anion exchange membranes (AEMs) provide one possible route to low platinum or platinum-free fuel cells with the potential for facile oxidation of complex fuels beyond hydrogen and methanol. AEMs and related stable cationic polymers also have applications in energy storage and other electrochemical technologies such as water electrolyzers and redox flow batteries. While anion exchange membranes have been known for a long time in water treatment applications, materials for electrochemical technology with robust mechanical properties in thin film format have only recently become more widely available. High hydroxide and bicarbonate anion conductivity have been demonstrated in a range of AEM formats, but intrinsic stability of the polymers and demonstration of long device lifetime remain major roadblocks. Novel approaches to stable materials have focused on new types of cations that employ delocalization and steric shielding of the positive center to mitigate nucleophilic attack by hydroxide. A number of promising polymer backbones and membrane architectures have been identified, but limited device testing and a lack of understanding of the degradation mechanisms in operating devices is slowing progress on engineered systems with alkaline fuel cell technology. Our objective is to spur more research in this area to develop fuel cell systems that approach the costs of inexpensive batteries for large-scale applications. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1727–1735, 2013     

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     

Highly Water Resistant Anion Exchange Membrane for Fuel Cells

For anion exchange membranes (AEMs), achieving efficient hydroxide conductivity without excessive hydrophilicity presents a challenge. Hence, new strategies for constructing mechanically strengthened and hydroxide conductive (especially at controlled humidity) membranes are critical for developing better AEMs. Macromolecular modification involving ylide chemistry (Wittig reaction) for the fabrication of novel AEMs with an interpenetrating polymer network structure is reported. The macromolecular modification is cost effective, facile, and based on a one‐pot synthesis. AEM water uptake is reduced to 3.6 wt% and a high hydroxide conductivity (69.7 mS cm⁻¹, 90 °C) is achieved simultaneously. More importantly, the membrane exhibits similar tensile strength (>35 MPa) and comparable flexibility in both dry and wet states. These AEMs could find further applications within anion exchange membrane fuel cells with low humidity or photoelectric assemblies.

     

Ion transport properties of mechanically stable symmetric ABCBA pentablock copolymers with quaternary ammonium functionalized midblock

Anion exchange membranes (AEMs) are a promising class of materials for applications that require selective ion transport, such as fuel cells, water purification, and electrolysis devices. Studies of structure–morphology–property relationships of ion‐exchange membranes revealed that block copolymers exhibit improved ion conductivity and mechanical properties due to their microphase‐separated morphologies with well‐defined ionic domains. While most studies focused on symmetric diblock or triblock copolymers, here, the first example of a midblock quaternized pentablock AEM is presented. A symmetric ABCBA pentablock copolymer was functionalized to obtain a midblock brominated polymer. Solution cast films were then quaternized to obtain AEMs with resulting ion exchange capacities (IEC) ranging from 0.4 to 0.9 mmol/g. Despite the relatively low IEC, the polymers were highly conductive (up to 60 mS/cm Br^? at 90 °C and 95%RH) with low water absorption (<25 wt %) and maintained adequate mechanical properties in both dry and hydrated conditions. X‐ray scattering and transmission electron microscopy (TEM) revealed formation of cylindrical non‐ionic domains in a connected ionic phase. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 612–622     

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     

Synthesis of ionic polybenzimidazoles with broad ion exchange capacity range for anion exchange membrane fuel cell application

In this study, new anion exchange membranes (AEM) based on crosslinked polybenzimidazole (m-PBI) with quaternary ammonium groups, crosslinkable allyl groups, and hydrophobic ethyl groups as side chains are synthesized and characterized. The AEMs are crosslinked by thermal thiol-ene reaction using a dithiol crosslinker. The ion exchange capacity (IEC) values and crosslinking density were controlled by the number of quaternary ammonium groups and allyl groups, respectively. The introduction of ethyl groups improved the solubility of ionic PBIs even at very low IEC values by eliminating the hydrogen bonding interaction of imidazole rings. This method allows ionic PBIs with broad IEC values, from 0.75 to 2.55 mmol/g, to be prepared. The broad IEC values were achieved by independently controlling the numbers of quaternary ammonium groups, allyl groups, and hydrophobic ethyl groups during preparation. The crosslinked ionic PBIs revealed hydroxide conductivity from 16 to 86 mS/cm at 80°C. The wet membranes also showed excellent mechanical strength with tensile strength of 12.2 to 20.1 MPa and Young's Modulus of 0.67 to 1.45 GPa. The hydroxide conductivity of a crosslinked membrane (0.40Q0.60Et1.00Pr, IEC = 0.95 mmol/g) decreased only 7.9% after the membranes was immersed in a 1.0 M sodium hydroxide solution at 80°C for 720 h. A single fuel cell based on this membrane showed a maximum peak power density of 136 mW/cm² with a current density of 377 mA /cm² at 60°C.     

Highly Conductive Anion‐Exchange Membranes from Microporous Tröger's Base Polymers

The development of polymeric anion‐exchange membranes (AEMs) combining high ion conductivity and long‐term stability is a major challenge for materials chemistry. AEMs with regularly distributed fixed cationic groups, based on the formation of microporous polymers containing the V‐shape rigid Tröger's base units, are reported for the first time. Despite their simple preparation, which involves only two synthetic steps using commercially available precursors, the polymers provide AEMs with exceptional hydroxide conductivity at relatively low ion‐exchange capacity, as well as a high swelling resistance and chemical stability. An unprecedented hydroxide conductivity of 164.4 mS cm^?1 is obtained at a relatively a low ion‐exchange capacity of 0.82 mmol g^?1 under optimal operating conditions. The exceptional anion conductivity appears related to the intrinsic microporosity of the charged polymer matrix, which facilitates rapid anion transport.     

Anion conducting multiblock copolymers with different tethered cations

A multiblock copoly(arylene ether) polymer was used to quantitatively compare the ion conducting channels formed by three different, tethered cation head‐groups. The synthesis allowed for the formation of an exact number of tethers on each repeat unit. Three head‐groups, quaternary trimethylammonium (TMA), quinuclidium (ABCO), and tris(2,4,6‐trimethoxyphenyl)phosphonium (TTMPP) cation head‐groups were compared in terms of size of the conducting channels, ionic conductivity of the mobile hydroxide ion, mechanical properties, quantity of productive and unproductive water, and chemical stability of the membrane in base. The interdomain spacing showed that multiblock copolymers with larger cations formed larger ion conduction channels in the membrane. Larger cations resulted lower ion exchange capacity (IEC) even though the polymer backbone and tether arrangements were identical. TMA was the most stable cation after exposure to 1 M NaOH at 60 °C for 20 days. ABCO had a lower number of bound water molecules and a 22% loss in ion conductivity after treatment in 1 M NaOH at 60 °C for 20 days due to the higher hydroxide ion concentration in the ion conductive blocks. Membranes with TMA head‐groups also had the best mechanical properties. Two membrane preparation methods were compared. The presence of the cation head‐groups assists in phase segregation. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1395–1403     

Understanding anion transport in an aminated trimethyl polyphenylene with high anionic conductivity

An alkaline exchange membrane (AEM) based on an aminated trimethyl poly(phenylene) is studied in detail. This article reports hydroxide ion conductivity through an in situ method that allows for a more accurate measurement. The ionic conductivities of the membrane in bromide and carbonate forms at 90 °C and 95% RH are found to be 13 and 17 mS cm⁻¹ respectively. When exchanged with hydroxide, conductivity improved to 86 mS cm⁻¹ under the same experimental conditions. The effect of relative humidity on water uptake and the SAXS patterns of the AEM membranes were investigated. SAXS analysis revealed a rigid aromatic structure of the AEM membrane with no microphase separation. The synthesized AEM is shown to be mechanically stable as seen from the water uptake and SAXS studies. Diffusion NMR studies demonstrated a steady state long-range diffusion constant, D_∞ of 9.8 × 10⁻⁶ cm² s⁻¹ after 50–100 ms. © 2012 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1743–1750, 2013     

Synthesis and characterization of benzimidazolium‐functionalized polysulfones as anion‐exchange membranes

Anion‐exchange membranes containing pendant benzimidazolium groups were synthesized from polysulfone by chrolomethylation followed by nucleophilic substitution reaction with 1‐methylbenzimidazole. The structures of the polymers were characterized by ¹H‐NMR and FTIR analysis. The resulting membranes showed high thermal stability below 200 °C. The values of water uptake and swelling degree increased with the ion‐exchange capacity of the polymeric membrane. The ionic conductivity was measured by means of impedance spectroscopy in aqueous solution of potassium hydroxide (10^?4?10^?1 M). The results show not only a clear correlation between the membrane's electrochemical behavior with the electrolyte solution embedded in the membrane, but also with the degree of the polysulfone's chloromethylation.Thus, the ionic conductivity increased more than two orders of magnitude when the degree of chloromethylation increased from 40 to 140%. Benzimidazolium‐functionalized polysulfones exhibited better thermal, mechanical, and electrochemical properties than the widely used polymeric membranes containing quaternary ammonium groups. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2363–2373     

基于阳离子交换基团的阴离子交换膜研究进展

近年来,阴离子交换膜燃料电池的发展受到了广泛关注。 开发具有碱稳定性能优异、电导率高的阴离子交换膜材料成为了研究的热点。 阴离子交换膜(AEM)主要由聚合物骨架和阳离子基团组成,除了聚合物骨架结构,离子交换基团是影响膜碱稳定性和电导率的重要因素,因此,设计离子基团是提高膜性能的重要手段之一。 本文综述了近年来功能基团分别为季铵、胍基、咪唑鎓盐、季鏻、金属配合物、N-螺环季铵盐、哌啶和吡咯等阳离子交换基团的AEM的研究进展,其中包括不同种类阳离子交换基团的AEM的结构,碱稳定性能和OH^-电导率,同时对于含有阳离子交换基团的AEM的结构设计进行了分析和展望。     

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.     

Synthesis and properties of hydroxide conductive polymers carrying dense aromatic side-chain quaternary ammonium groups

A series of hydroxide conductive polymers QTBMs carryingdense aromatic side-chain quaternary ammonium groups has been synthesized by using a new monomer of 3,3′-di(3′′,5′′-dimethylphenyl)-4,4′-difluorodiphenyl sulfone and other commercial monomers via polycondensation reaction, and subsequent bromination, quaternization and alkalization.The chemical structures of the ionomers were confirmed by ~1H-and ~(13)C-NMR spectroscopy. Water uptake, swelling ratio, hydroxide conductivity, the number of bonded water per ammonium group(λ), volumetric ion exchange capacity(IEC_(Vwet)), mechanical and thermal properties, and chemical stability were systematically evaluated for the series of QTBMs membranes. QTBMs showed IECs ranging from1.02 meq·g~(-1)to 2.11 meq·g~(-1); in particular, QTBM-60 membrane with the highest IEC(2.11 meq·g~(-1)) had very high hydroxide ion conductivity of 131.9 m S·cm~(-1) at 80 °C, which was attributed to the well assembled nano-channels with distinct phase separation evidenced by small-angle X-ray scattering(SAXS). It was found that the hydrated QTBMs membranes were mechanically stable with moderate water uptakes and swelling ratios, high chemical stability under the harsh alkaline conditions. This work provides a facile way to prepare anion exchange membranes(AEMs) with high performances for the application in alkaline fuel cells.     

Highly Stable,Low Gas Crossover,Proton‐Conducting Phenylated Polyphenylenes

Two classes of novel sulfonated phenylated polyphenylene ionomers are investigated as polyaromatic‐based proton exchange membranes. Both types of ionomer possess high ion exchange capacities yet are insoluble in water at elevated temperatures. They exhibit high proton conductivity under both fully hydrated conditions and reduced relative humidity, and are markedly resilient to free radical attack. Fuel cells constructed with membrane‐electrode assemblies containing each ionomer membrane yield high in situ proton conductivity and peak power densities that are greater than obtained using Nafion reference membranes. In situ chemical stability accelerated stress tests reveal that this class of the polyaromatic membranes allow significantly lower gas crossover and lower rates of degradation than Nafion benchmark systems. These results point to a promising future for molecularly designed sulfonated phenylated polyphenylenes as proton‐conducting media in electrochemical technologies.     

Direct Insertion Polymerization of Ionic Monomers: Rapid Production of Anion Exchange Membranes

The limited number of methods to directly polymerize ionic monomers currently hinders rapid diversification and production of ionic polymeric materials, namely anion exchange membranes (AEMs) which are essential components in emerging alkaline fuel cell and electrolyzer technologies. Herein, we report a direct coordination-insertion polymerization of cationic monomers, providing the first direct synthesis of aliphatic polymers with high ion incorporations and allowing facile access to a broad range of materials. We demonstrate the utility of this method by rapidly generating a library of solution processable ionic polymers for use as AEMs. We investigate these materials to study the influence of cation identity on hydroxide conductivity and stability. We found that AEMs with piperidinium cations exhibited the highest performance, with high alkaline stability, hydroxide conductivity of 87 mS cm⁻¹ at 80 °C, and a peak power density of 730 mW cm⁻² when integrated into a fuel cell device.