Types Of Anion Exchange Membranes Research Articles

Chemical stability of anion exchange membranes for alkaline fuel cells and water electrolysers: Experiment, multi-dimensional modeling and hybrid simulations

The unstable nature of anion exchange membranes (AEMs) has hindered the progress of alkaline fuel cells and water electrolyzers. Density functional theory (DFT) can explore the correlation between the structure and properties of model compounds. However, theoretical explorations of AEMs remain immensely insufficient owing to the complexity of modeling the realistic solution system which caused by multi-dimensional parameters such as polymer chain entanglement, steric hindrance, and migration of ions. In this study, molecular dynamic simulation was utilized to obtain the multi-dimensional parameters of polyelectrolytes, which were considered when calculating the degradation energy barrier by DFT, thus leading to more accurate results and optimizing the conventional DFT calculations. Combined with the computational works, pyridinium functionalized polyolefin and polyether sulfone were synthesized to verify the feasibility of the simulation scheme. This optimized scheme fills the gap in the DFT calculations scale and is easy to extend to other types of AEMs, which help design the chemically stable AEMs and promote the development of alkaline fuel cells and water electrolysers.

Understanding the Role of Cu Electocatalyst, Binder, and Membrane Interfaces in CO2 Electrolyzer Durability

Carbon dioxide electrolyzers capable of maintaining stable performance for thousands of hours are necessary for commercial electrochemical reduction of CO2 to two-carbon products such as ethanol and ethylene. Currently, most studies containing CO2 reduction to two-carbon products have been for durations of under 20h. Only a few (<5) instances of CO2 reduction to two-carbon products at durations over 100h have been reported. Common cell failure mechanisms, such as carbonate salt precipitation and cathode flooding, can be mitigated by tuning the operating conditions of the reactor. Beyond these modes of failure, there are practical limitations related to materials' durabilities and long-term stabilities, including ionomeric membranes' operating lifetimes, gas diffusion cathodes, anodes, and other MEA components.In this presentation, we explore the role of CO2, water, and ion transport on cell performance over extended periods of operation. The polymeric anion exchange membrane and conductive binder have been identified as crucial components in tuning the transport properties of electrolyzer systems and, thus, will be the focus of this study. We consider several types of anion exchange membranes (e.g., imidazolium, ammonium, piperindinium) and conductive binders (e.g., Nafion, PVDF, PTFE, PSMIM) for use in the direct reduction of CO2 to two-carbon products at nanoscale Cu electrodes (150mA/cm2, 0.01M KHCO3 anolyte). This work improves upon previous analysis of membrane and binder transport properties by testing a wider variety of materials for >100h durations and reporting specific cell failure times and causes. We will discuss the balances between carbonate precipitation, flooding, and other membrane or electrode failures as a function of materials, MEA preparation, and operating conditions. A comprehensive set of failure standards based on electrolyzer performance (pressure, outlet mass flow, two-carbon product Faradaic efficiencies, energy efficiency, carbon efficiency) is introduced, and electrolyzer configurations are tested in triplicate to produce statistically significant failure data. Overall, this provides insight into CO2, ion, and water transport's role in electrolyzer cell performance and failure mechanisms. Figure 1

Effects of the crown ether cavity on the performance of anion exchange membranes

Anion exchange membrane fuel cells (AEMFCs) have emerged as a promising alternative to proton exchange membrane fuel cells (PEMFCs) due to their adaptability to low-cost stack components and non-noble-metals catalysts. However, the poor alkaline resistance and low OH− conductivity of anion exchange membranes (AEMs) have impeded the large-scale implementation of AEMFCs. Herein, the preparation of a new type of AEMs with crown ether macrocycles in their main chains via a one-pot superacid catalyzed reaction was reported. The study aimed to examine the influence of crown ether cavity size on the phase separation structure, ionic conductivity and alkali resistance of anion exchange membranes. Attributed to the self-assembly of crown ethers, the poly (crown ether) (PCE) AEMs with dibenzo-18-crown-6-ether (QAPCE-18-6) exhibit an obvious phase separated structure and a maximum OH− conductivity of 122.5 mS cm−1 at 80 °C (ionic exchange capacity is 1.51 meq g−1). QAPCE-18-6 shows a good alkali resistance with the OH− conductivity retention of 94.5% albeit being treated in a harsh alkali condition. Moreover, the hydrogen/oxygen single cell equipped with QAPCE-18-6 can achieve a peak power density (PPD) of 574 mW cm−2 at a current density of 1.39 A cm−2.

Understanding the Effect of Triazole on Crosslinked PPO-SEBS-Based Anion Exchange Membranes for Water Electrolysis.

For anion exchange membrane water electrolysis (AEMWE), two types of anion exchange membranes (AEMs) containing crosslinked poly(phenylene oxide) (PPO) and poly(styrene ethylene butylene styrene) (SEBS) were prepared with and without triazole. The impact of triazole was carefully examined. In this work, the PPO was crosslinked with the non-aryl ether-type SEBS to take advantage of its enhanced chemical stability and phase separation under alkaline conditions. Compared to their triazole-free counterpart, the crosslinked membranes made with triazole had better hydroxide-ion conductivity because of the increased phase separation, which was confirmed by X-ray diffraction (XRD) and atomic force microscopy (AFM). Moreover, they displayed improved mechanical and alkaline stability. Under water electrolysis (WE) conditions, a triazole-containing crosslinked PPO-SEBS membrane electrode assembly (MEA) was created using IrO2 as the anode and a Pt/C catalyst as the cathode. This MEA displayed a current density of 0.7 A/cm2 at 1.8 V, which was higher than that of the MEA created with the triazole-free counterpart. Our study indicated that the crosslinked PPO-SEBS membrane containing triazoles had improved chemo-physical and electrical capabilities for WE because of the strong hydrogen bonding between triazole and water/OH-.

Micro-block poly(arylene ether sulfone)s with densely quaternized units for anion exchange membranes: Effects of benzyl N-methylpiperidinium and benzyl trimethyl ammonium cations

The piperidinium cation exhibited a substantial impact on the improvement of alkaline stability of anion exchange membranes (AEMs), yet the research regarding the micro-block polymer AEMs modified with it has not been conducted. Herein, we report two series of micro-block poly(arylene ether sulfone)s containing densely benzyl-N-methylpiperidinium (TTP-n-PIPPES) and benzyl-trimethyl-ammonium (TTP-n-QAPES) functionalized units for AMEs, respectively, where the n represents the molar ratio of monomer containing tri-tetraphenyl to the employed bisphenol monomers. The content and arrangement of the two types of cations in both polymer backbones were the same. The hydrophilic phases of TTP-n-PIPPES membranes exhibited isolated island states, wrapped with coherent hydrophobic phases. While the TTP-n-QAPES membranes formed continuous hydrophilic channels but fragmented hydrophobic phases. The discrepancy in the microscopic morphology of membranes and the intrinsic nature of cations thus rendered two types of AEMs with different advantages in macroscopic properties. The TTP-n-PIPPES membranes performed better dimensional stability and alkaline stability (low temperature). In contrast, the preferable capabilities of TTP-n-QAPES membranes were water affinity, hydroxide conductivity, and alkaline stability (high temperature). For instance, the TTP-18.2-PIPPES membrane was 11.7%, 11.4%, and 27.4% lower than the TTP-18.2-QAPES membrane at 80 °C in terms of dimensional change, water uptake, and hydroxide conductivity, respectively.

Investigation of Transport Processes through Ion-Exchange Membranes Used in the Production of Amines from Their Salts Using Bipolar Electrodialysis.

The influence of the nature of amine solutions on the frequency spectrum of the electrochemical impedance of the bipolar membrane aMB-2m is investigated. Moreover, the effect of the circulation rate of solutions in the electrodialyzer chambers on the volt-ampere characteristics of the Ralex AMH and MA-40L anion-exchange membranes and the aMB-2m bipolar membrane has been investigated. The diffusion characteristics of various types of anion-exchange membranes in a system containing dimethylammonium sulfate ((DEA)2H2SO4), as well as the diffusion characteristics of the Ralex AMH membrane in systems with methylammonium sulfate, dimethylammonium sulfate, diethylammonium sulfate, and ethylenediammonium sulfate ((MA)2H2SO4, (DMA)2H2SO4, (DEA)2H2SO4, EDAH2SO4) have been studied. It is shown that diffusion permeability depends on the structure and composition of anion-exchange membranes, as well as on the nature of amines. The technical and economic characteristics of the electromembrane processes for the production of amines and sulfuric acid from amine salts are determined. It is shown that when using Ralex AMH anion-exchange membranes in an electrodialyzer together with bipolar aMB-2m membranes, higher concentrations of diethylamine and sulfuric acid are achieved, compared with the use of MA-40L anion-exchange membranes.

Preparation and characterization of organic-inorganic hybrid anion exchange membrane based on crown ether functionalized mesoporous SBA-NH2

In this paper, self-made diformyl-dibenzo-18-crown-6 ether (DDB 18 C 6 ) is in-situ grafted into the pores of mesoporous molecular sieve (SBA-NH 2 ), and then the aforementioned modified molecular sieve (SBA-C) is introduced into the polyvinyl alcohol solution, and then the glutaraldehyde as the crosslinking agent to synthesized the anion exchange membrane under for fuel cell application. During the experiment, a series of anion exchange membranes (P-(SBA x% -C), x is the mass fraction of SBA-NH 2 ) is developed and the pore channels and chemical structures of the aforementioned membrane is verified by FT-IR, SAXD, N 2 adsorption-desorption, 1 H NMR and XPS. Moreover, the performance of the membrane synthesized in this paper is also investigated and the results revealed that the unique membrane internal structure can improve the OH − transportation efficiency. Furthermore, the ionic conductivity of P-(SBA 10% -C) membrane is the highest (0.107S·cm −1 ) and the power density is the highest (354.8 mW cm −2 ) at 80 °C. By immersing P-(SBA 10% -C) membrane in 6 mol L −1 KOH solution for 168 h, the conductivity at 80 °C only decreased by 2%, proving that P-(SBA 10% -C) has a higher conductivity, good single cell performance and alkali stability. • A new kind of mesoporous molecular sieve with ordered pores grafted with crown ether was prepared. • A new type of anion exchange membrane with ordered mesoporous channels grafted with crown ether was prepared. • Using crown ether to complex K + to improve the conductivity of anion exchange membranes. • Prepared membrane showed excellent OH − conductivity and alkaline stability.

Strategies to increase ion selectivity in electrodialysis

We analyze selective removal of anions in electrodialysis (ED) from a multicomponent ion mixture where all ions are monovalent, both experimentally, and by a modified theory. The theory makes use of the Nernst-Planck equation to describe transport of ions across ion-exchange membranes in combination with the ion affinity, which is a parameter that accounts for the preference of membrane materials to adsorb one ion more than another. For a mixture of NO3- and Cl- anions, ion affinity was measured in an adsorption experiment, and it was found that NO3- adsorbs more in anion-exchange membranes (AEMs) than Cl-. Faster transport of NO3- relative to Cl- in batch-mode ED experiments is well described by the transport theory, for three types of AEMs that we tested. From analysis of the model, we can derive that membrane selectivity in ED between different anions can be increased further by: i) an increase in the difference in ion affinity of the anions, ii) an increase in the AEM thickness, and iii) a decrease in the charge density of the AEM. The last two strategies go against the general understanding that thin highly charged membranes are in general better for ED. Our proposed strategies follow rigorously from our combined theoretical–experimental study.

Poly(fluorenyl ether ketone)s with densely distributed multi-cationic side chains for anion exchange membranes

The anion conductivity is one of the key properties of anion exchange membranes (AEMs), which is affected by the content and distribution of ionic groups in the ionomers. Herein, a series of poly(fluorenyl ether ketone)s with multiple phenylmethyl groups were synthesized. After bromination, the phenylmethyl groups were attached with linear tri- and di-quaternary ammonium side chains. As a result, the quaternary ammonium groups were tethered closely at the aliphatic side chains which emanate from the specific segments of the aromatic backbone. The ion exchange capacity, water affinity, Br− conductivity, morphology, thermal property, mechanical property, oxidative stability, and VO2+ permeability of the obtained AEMs were studied. It has been found that both types of AEMs have well-developed hydrophilic-hydrophobic phase separation. When comparing at similar ion exchange capacity (IEC), the AEMs bearing tri-quaternary ammonium side chains have higher Br− conductivity than the AEMs bearing di-quaternary ammonium side chains, presumably because of the better phase separation in the former. All of the AEMs have significantly reduced VO2+ permeability than Nafion 212, owing to the positive effect of Donnan repulsion. Along with the other characterizations, the obtained AEMs were proven to be promising candidates for potential vanadium redox flow battery applications.

Transport and Co-Transport of Carboxylate Ions and Ethanol in Anion Exchange Membranes.

Understanding multi-component transport behavior through hydrated dense membranes is of interest for numerous applications. For the particular case of photoelectrochemical CO2 reduction cells, it is important to understand the multi-component transport behavior of CO2 electrochemical reduction products including mobile formate, acetate and ethanol in the ion exchange membranes as one role of the membrane in these devices is to minimize the permeation of these products. Anion exchange membranes (AEM) have been employed in these and other electrochemical devices as they act to facilitate the transport of common electrolytes (i.e., bicarbonates). However, as they act to facilitate the transport of carboxylates as well, thereby reducing the overall performance, the design of new AEMs is necessary to improve device performance through the selective transport of the desired ion(s) or electrolyte(s). Here, we investigate the transport behavior of formate and acetate and their co-transport with ethanol in two types of AEMs: (1) a crosslinked AEM prepared by free-radical copolymerization of a monomer with a quaternary ammonium (QA) group and a crosslinker, and (2) Selemion® AMVN. We observe a decrease in diffusivities to carboxylates in co-diffusion. We attribute this behavior to charge screening by the co-diffusing alcohol, which reduces the electrostatic attraction between QAs and carboxylates.

Mechanically robust microporous anion exchange membranes with efficient anion conduction for fuel cells

• Brittleness in polymer of intrinsic microporosity (PIM) membranes is alleviated by high molecular weight. • Role of micropores in developing water/ion conduction channels is investigated. • Microporous anion exchange membrane (AEM) has superior ion conduction efficiency. • H 2 crossover through microporous AEM is minimal in the hydrated state. • Feasibility of microporous AEMs for H 2 /O 2 fuel cell is evaluated. Polymers of intrinsic microporosity (PIMs) present an attractive opportunity for developing new types of anion exchange membranes (AEMs) for fuel cell featuring charged subnanometer-sized micropores. But challenges exist to make mechanically robust PIM AEMs due to their high chain rigidity. Imparting more flexibility improves mechanical properties but sacrifices microporosity. Here, a mechanically robust and highly anion conductive PIM AEM (QPIM-1) fabricated by facile animation and quaternization of PIM-1 membrane is reported, and its structure–property relationships are investigated, especially focusing on the microporous structure. High molecular weight alleviates brittleness, as QPIM-1 AEM shows comparable mechanical properties to conventional AEMs, quaternized poly(2,6-dimethyl-1,4-phenylene oxide) (QPPO), at a membrane thickness down to ~35 μm and a high ion exchange capacity (IEC) up to ~2.1 mmol g −1 . The micropores situated among the rigid and contorted polymer chains evolve into water/ion conduction channels when the membrane is hydrated. This results in improved morphology over dense polymeric AEMs by less hindered ion pathways. QPIM-1 AEMs exhibit superior ion conduction efficiency, which is 2.6–5.3 times that of dense QPPO AEM at similar ion exchange capacities (IECs). A high hydroxide ion conductivity of 57 mS cm −1 at 20 °C is obtained, which is among the highest reported anion conductive PIM-based AEMs. Even though the AEMs are microporous, only slight H 2 permeation is observed when hydrated and at high open circuit voltage (OCV) of a single fuel cell.

Highly conductive anion exchange membranes based on polymer networks containing imidazolium functionalised side chains

Two novel types of anion exchange membranes (AEMs) having imidazolium-type functionalised nanofibrous substrates were prepared using the facile and potentially scalable method. The membranes’ precursors were prepared by graft copolymerization of vinylbenzyl chloride (VBC) onto syndiotactic polypropylene (syn-PP) and polyamide-66 (PA-66) nanofibrous networks followed by crosslinking with 1,8-octanediamine, thermal treatment and subsequent functionalisation of imidazolium groups. The obtained membranes displayed an ion exchange capacity (IEC) close to 1.9 mmol g–1 and ionic (OH-) conductivity as high as 130 mS cm–1 at 80 °C. This was coupled with a reasonable alkaline stability representing more than 70% of their original conductivity under accelerated degradation test in 1 M KOH at 80 °C for 360 h. The effect of ionomer binder on the performance of the membrane electrode assembly (MEA) in AEM fuel cell was evaluated with the optimum membrane. The MEA showed a power density of as high as 440 mW cm−2 at a current density is 910 mA cm−2 with diamine crosslinked quaternized polysulfone (DAPSF) binder at 80 °C with 90% humidified H2 and O2 gases. Such performance was 2.3 folds higher than the corresponding MEA performance with quaternary ammonium polysulfone (QAPS) binder at the same operating conditions. Overall, the newly developed membrane was found to possess not only an excellent combination of physico-chemical properties and a reasonable stability but also to have a facile preparation procedure and cheap ingredients making it a promising candidate for application in AEM fuel cell.

Anion exchange membranes with eight flexible side-chain cations for improved conductivity and alkaline stability

Increasing the local charge density of flexible side-chain cations in the hydrophilic segments of anion exchange membranes (AEMs) is helpful for improving their properties. However, due to limitations of structural design strategies and available synthetic methods, very few AEMs with more than four flexible side-chain cationic groups in hydrophilic segments have been reported. In order to further improve the hydroxide conductivity, alkaline stability and dimensional stability, herein we report a series of AEMs containing eight flexible side-chain cations in hydrophilic segments, based on poly(aryl ether sulfone)s (PAES). The synthesis, ion exchange capacity (IEC), water absorption, dimensional swelling, alkaline stability and hydroxide conductivity of the obtained membranes (PAES-8TMA-x) were examined and the relationships between structures and properties of different types of AEMs were also systematically compared. The resulting AEMs with IEC values of 1.76–2.76 mmol g−1 displayed comprehensively desirable properties, with hydroxide conductivities of 62.7–92.8 mS cm−1 and dimensional swelling in the range of 8.3% to 15.8% at 60°C. The IEC and hydroxide conductivity for a representative sample, PAES-8TMA-0.35, maintained 82.2% and 79.6% of the initial values after being immersed in 2 mol L−1 NaOH at 90°C for 480 h, respectively. This study expands the design and preparation of AEMs containing high local densities of flexible side chain cations, and provides a new strategy for new AEM materials.

Allyl group-enabled side chain grafting for anion exchange membrane fabrication

In this paper, we report a novel type of anion exchange membrane, which was synthesized by polysulfone modification with N-methyl diallylamine, whose allyl group enabled subsequent graft copolymerization of 4-vinylbenzyl chloride (VBC) and styrene (ST); quaternization was achieved with N-methylmorpholine (Mm). By controlling the amount of VBC and ST, a series of grafted side chain-type anion exchange membranes, MmOH-PSf-VS-x (x standing for VBC content), was prepared. Attributed to good microphase separation, the MmOH-PSf-VS-12 membrane exhibited a high conductivity of 33.71 mS cm−1 at 20 °C, which is 34% higher than that of the N-methylmorpholine quaternized polysulfone membrane without side chains (MmOH-PSf) at a similar ion exchange capacity (IEC). Moreover, the swelling resistance and mechanical properties of the membranes were also improved with the side chain grafting. Our work provides a new route to fabricate side chain grafted anion exchange membrane (AEM).

Effect of CO2 on the properties of anion exchange membranes for fuel cell applications

In this study the effect of CO2, HCO3‾ and CO32‾ on the ionic conductivity and water uptake properties of anion exchange membranes (AEMs) was investigated in order to better understand the detrimental effect of ambient air feed on the performance of AEM fuel cells. Three types of AEMs were examined, including Poly(hexamethyl-p-terphenyl benzimidazolium) (HMT-PMBI), Fumatech® FAA-3, and poly(phenylene oxide) functionalized with imidazole (PPO-Im). The effect of temperature and humidity on AEM properties in their different anion forms was studied, including both steady state and dynamic measurements. In addition, the response to changes in CO2 concentration and to application of ex-situ electric current was examined. Results showed that an increase in humidity leads to an increase in water content and an increase in conductivity of the AEMs, regardless the anion type. It was found that both temperature and relative humidity improve conductivity in carbonated forms, however relative humidity has the most significant impact. The carbonation process in 400 ppm CO2 is slightly quicker in AEMs with low conductivity, lasting ca. 40 min; however it was shown that a reverse process can be achieved by applying an electric current through the AEMs. An increase by 2–10 fold in conductivity is obtained using this method, which is analogous to the changes observed during operation of the fuel cell. This work provides important data that needs to be taken into account in future work in order to ultimately mitigate the carbonation effects and improve the performance of AEM fuel cells running with ambient air.

Integration of Bipolar Membrane Electrodialysis with Ion-Exchange Absorption for High-Quality H3PO2 Recovery from NaH2PO2.

H3PO2 has emerged as an indispensable reducing agent for electroless nickel plating. Commercial preparation of H3PO2, with high purity and low cost, is a great challenge. In this work, a novel technique by the integration of bipolar membrane electrodialysis (BMED) with ion-exchange absorption was designed to prepare high-quality H3PO2 aqueous solution. The critical parameters, such as voltage drop, NaH2PO2 concentration, and different types of anion-exchange membranes, were systematically investigated. Continuous experiments indicated that a high yield of up to 80.06% with a low energy consumption of 4.99 kW h/kg was achieved under optimal operation conditions (voltage drop of 20 V, feed concentration of 15 wt % NaH2PO2, and anion-exchange membrane of AHA). Moreover, leakage of Na+ ions through the bipolar membrane was observed. By using T-52H cation-exchange resin, the final concentration of Na+ ions in H3PO2 aqueous solution was reduced to 20.91 mg/L. Subsequently, a long-term experiment was performed to evaluate the stability of the BMED stack, and the concentration of H3PO2 in the acid compartment reached 4.15 mol/L. Under optimal conditions, the H3PO2 production cost was estimated at $0.937 kg–1, which was competitive and economically friendly for industrial application.

Hybrid capacitive deionization with anion-exchange membranes for lithium extraction

Lithium is considered to be a critical material for various industrial fields. We present our studies on extraction lithium from diluted aqueous solution by novel hybrid system based on a membrane capacitive deionization and batteries desalination. Hybrid CDI is comprised by a lithium selective adsorbent, activated carbon electrode and anion-exchange membranes. Here, we demonstrated implication of various type of anion-exchange membranes and influence their properties on effective capacity and energy requirements in charge/discharge steps. We described a configuration with anion-exchange membrane characterized by adsorption capacity of 35 mg/g of Li + with 0.08Wh/g and removal efficiency of 60 % of lithium ions, using novel selective desalination technique.

Preparation and characterization of imidazolium-based membranes for anion exchange membrane fuel cell applications

New anion exchange membranes (AEMs) with high conductivity, good dimensional and alkaline stability are currently required in order to develop alkaline fuel cells into efficient and clean energy conversion devices. In this study, a series of AEMs based on 1, 2-dimethyl-3-(4-vinylbenzyl) imidazolium chloride ([DMVIm][Cl]) are prepared and investigated. [DMVIm][Cl] is synthesized and used as ion carriers and hydrophilic phase in the membranes. The water uptake, swelling ratio, IEC and conductivity of the AEMs increase with increasing the [DMVIm][Cl]. The imidazolium-based AEMs show excellent thermal stability, sufficient mechanical strength, the membrane which containing 30% mass fraction of [DMVIm][Cl] shows conductivity up to 1.0 × 10−2 S cm−1 at room temperature and good long-term alkaline stability in 1 M KOH solution at 80 °C. The results of this study suggest that this type of AEMs have good perspectives for alkaline anion exchange membrane fuel cell applications.

The removal of fluoride from water based on applied current and membrane types in electrodialyis

In many places over the world, the fluoride concentration in ground water as raw water is higher than the threshold standard for healthy drinking water. This paper discusses a fluoride ion removal from sodium fluoride solution by using batch-system electrodialysis (ED) process. The effect of type of anion exchange membranes on the deionization rate of fluoride ion is investigated. The removal of fluoride ion from water by electrodialysis using CMX/AMX and CMX/ACS cation/anion exchange membranes combinations has been investigated on various applied currents, such as 3.64, 5.45, 9.09, 12.7, and 18.2 (A/m2). It was found that the increase of current densities made an operational time of ED shorter to achieve maximal ion removal. The effect of other coexistent ions such as chloride ion in water on the profile of ion removal and the ED cell resistance was discussed. It was also found that chloride ion passed more easily through anion exchange membranes than through fluoride ion.

Alkaline-Stable Anion Exchange Blend Membranes (AEBMs) with a Novel Sterically Hindered Cationic Head Group

In this study, synthesis and characterization of Anion Exchange Blend Membranes (AEBMs) are introduced. AEBMs were prepared by mixing polymer solutions of a halomethylated polymer (Br-PPO, brominated poly(2,6-dimethyl-1,4-phenylene oxide)), polybenzimidazole (PBI, F6PBI or PBI-OO) and a sulfonated polymer (S-polymer, SAC 098 (a sulfonated poly(phenylethersulfone), nonfluorinated) or SFS 001 (a sulfonated aromatic polyether, partially fluorinated)). A halomethylated polymer was used as the anion exchange ionomer precursor. Polybenzimidazole was used as a matrix polymer in order to enhance the mechanical strength of the AEBN. A minor amount of sulfonated polymer was added, forming ionic cross-links, and covalent bonds were formed in the blend membrane by reaction of a small amount of the CH2Br groups of Br-PPO with imidazole N-H groups of the PBI, respectively, leading to an increase of the chemical and dimensional stability of the novel AEBMs [1]. Membranes were fabricated by following procedure 1) mixing of the polymer solutions to homogeneity, 2) solvent evaporation in oven, 3) quaternization of the blend membrane by soaking the membrane in amine solution, 4) Ion exchange into chloride form in sodium chloride solution. It is well known that many types of anion exchange membranes are chemically unstable under alkaline condition. S. Holdcroft published a novel polybenzimidazolium AEM with a mesitylene building block shielding the dimethylimidazolium moiety of the polybenzimidazolium from OH- attack, leading to excellent hydroxide stability of this AEM: after immersion of this AEM in 2M KOH for 10 days at 60°C, no significant change of the NMR spectrum was observed [2]. Therefore it can be concluded that introduction of bulky steric hindrance groups in the vicinity of the cationic groups of AEMs can prevent the quaternized amine in the polymer structure from nucleophilic attack of hydroxide ions. Consequently in this study several types of sterically hindered tertiary amines were introduced in the polymer blend system for quaternization with Br-PPO, among them 1,2,2,6,6-pentamethylpiperidine (pempidine), quinuclidine, and 1,2,4,5-tetramethyl-1H-imidazole (abbreviated TMIm). Among these AEBMs, the TMIm-based membranes showed the best properties in terms of conductivity and stability. We investigated different combinations of polymers for AEBMs. The membrane containing PBI-OO and SAC098 showed good weight maintenance than other material combinations in DMAc extraction test at 90oC. This membrane was prepared and post-treated with TMIm as mentioned above. The membrane exhibited a chloride conductivity of 6.5 mS/cm at 30oC and 90% RH which is higher than that of commercial membrane (Tokuyama A202) under the same condition. The change of ion exchange capacities (IECs) was also investigated in order to examine the hydroxide stability. The membrane was exposed to 1M KOH solution at 90oC for 10days, and ion-exchange capacities (IECs) were compared to each other before and after KOH treatment. The IECs before and after KOH treatment were 2.63 and 2.59 mmol/g respectively, indicating excellent hydroxide stability. Apart from Br-PPO, other halomethylated polymers such as polyvinylbenzylchloride and polyepichlorohydrine were also investigated as precursors for AEBMs in this study, yielding good Cl- conductivities (in excess of 30 mScm-1) and alkaline stabilities with the novel 1,2,4,5-tetramethyl-1H-imidazolium head group. Addition of a hydrophilic polymer phase to the novel AEBMs resulted in even higher Cl- conductivities (e. g. 131 mScm-1@90°C@90% r.h.) after 10 d of KOH treatment (1M, 90°C).