Properties Of Anion Exchange Membranes Research Articles

Advances in alkaline stable guanidinium based anion exchange membranes

Anion exchange membranes (AEMs) are solid polymer electrolytes that contain cationic groups covalently bound to or embedded in a polymer backbone. They act as key role of hydroxide conduction and separator between two electrodes in anion exchange membrane fuel cells (AEMFCs). To achieve high performance in AEMFCs, ideal AEMs should own high ion conductivity and good alkaline stability. Considering to the alkaline stability of AEMs, ionic groups play very important role in determining the overall stability for the AEMs apart from polymer backbones. To date, various ion conductive moieties have been investigated as new tethered ionic groups to replace conventional quaternary ammoniums (QAs) in AEMs because of the multiple degradation pathways under alkaline conditions including: (1) nucleophilic substitution (SN2), (2) Hoffmann elimination (E2), or (3) nitrogen ylide formation. Guanidiniums are found to be a kind of novel ionic groups for the application of AEMs. In the structure of guanidinium, the positive charge is uniformly distributed over the central carbon atom and three nitrogen atoms, leading to stabilized charge delocalization and good thermal and chemical stability. There are six substitutions distributed over three N atoms which can be easily tuned by a mature and feasible route. Additionally, the kinetics of hydrogen evolution/oxidation reaction (HER/HOR) and ORR catalyzed by Pt and carbon-based non precious metal could be greatly enhanced through using guanidiniums. Guanidinium cations can be introduced into the polymer backbone easily by the synthetic methods as follows: (1) halomethylation of a phenyl group and subsequent amination with pentalkylguanidine; (2) nucleophilic substitution of a Vilsmeier salt and a secondary amine; (3) activated fluoroamine reaction; (4) nucleophilic substitution of alkyl halid and guanidine to prepare guanidinium functional monomer; (5) polycondensation between guanidine hydrochloride and diamine. Recently, several types of guanidinium-functionalized AEMs have been explored including benzylic guanidinium AEMs, alkylic guanidinium AEMs and phenylic guanidinium AEMs. Some of the AEMs have been proven to possess excellent ion conductivity, and some have been used in AEMFCs and show good electrochemical performance. Considering to degradation mechanism of guanidinium, it can be concluded as a nucleophilic addition-elimination reaction: firstly the center C atom is attacked by OH- and an intermediate is obtained (addition reaction); then one of N atoms combines with the H atom and eliminates from the intermediate (elimination reaction); finally the degradation products urea and amine are formed. It is worth noting that the degradation process of guanidinium is evidently different from quaternary ammoniums. Their degradation reaction sites only occur at the central carbon atom rather than the benzyl carbon which indicate the promising stability for guanidinium. Though the stabilities of guanidiniums are under debate by now, the usability of guanidiniums as ionic group is still a topic worthy of study. On the one hand, almost all of the studies are confined to guanidiniums with methyl groups substituted at the N1 and N3 positions, hence comprehensive and systematic analysis of the structure-stability relationship of guanidinium cations in alkaline media is still rarely developed. On the other hand, the degradation process of guanidiniums is only speculated by a few studies in theoretical way, more specific experimental studies of this process are badly needed to prove it. Therefore, revealing the structure-stability relationship and the detailed degradation mechanism are vital to design alkaline stable guanidinums which can be applied to AEMs preparation in the future. In addition, the polymer backbone is also very important to improve the stability of guanidiniums based AEMs. More guanidiniums based AEMs with various structures should be designed and prepared. The ion conductivity, alkaline stability and cell performace should be assessed comprehensively to figure out the relationship between the structures and properties of AEMs, and finally the preparation of high performance anion exchange membranes can be achieved.

The Effect of Concentration and pH of NaCl Solution on the Transport Properties of Anion Exchange Membranes with Different Fixed Groups

The effect of pH and concentration of the NaCl solution on the exchange capacity and transport properties of the MA-40 and MA-41 anion-exchange heterogeneous membranes with different nature of fixed groups has been studied: MA-40 has weakly basic groups, and MA-41 has strongly basic groups with a small proportion of weakly basic ones. The exchange capacity, thickness, water content, and electrical conductivity of the membranes equilibrated with NaCl solutions of various concentrations and pH were measured; for the same solutions, the diffusion permeability coefficients were found. The capacity was measured in the pH range from 1.5 to 12; the remaining properties, in the pH range from 3 to 9. Using the values of electrical conductivity and diffusion permeability, the ion transport numbers in the membranes were calculated. It was shown that at the external solution pH 9 the thickness of the membranes and their electric conductivity are minimal, and the transport numbers of co-ions are maximum. This is explained by the fact that in basic solutions weakly basic functional groups are largely deprotonated, and the effective capacity of the membrane is significantly reduced. The maximum effective capacity is achieved at pH $$ \leqslant $$ 3; in this case, transport numbers of co-ions in the MA-40 membrane are 5-fold, and in the MA-41 membrane, two-fold lower than corresponding values at pH 6 and 9. The changes in the transport properties of the membranes with increasing pH are due to a decrease in the degree of protonation of weakly basic functional groups, these changes are more pronounced for the MA-40 membrane than for MA-41.

Preparation of anion exchange membrane with branch polyethyleneimine as main skeleton component

A series of anion exchange membranes (AEMs) consisting of polyvinyl alcohol/branched polyethyleneimine (PVA/BPEI) were simply prepared by casting method using glutaraldehyde (GA) as crosslinking agent, then quaternized with benzyl chloride and alkalized in KOH solution. Subsequently, the comparative analysis was performed to select the optimal ratio of PVA to BPEI and crosslinking degree of AEMs. The structure and properties of AEMs were verified by FT-IR, XRD, TGA, mechanical analysis, water uptake test, swelling ratio test, ion exchange capacity (IEC) test, ionic conductivity measurement, and alkaline/oxidative stability test. The results showed that the maximum value of ionic conductivity was 86.0 mS/cm at 80 °C with IEC value of 2.35 meq/g. Moreover, the AEM had excellent chemical stability, as the ionic conductivity decreased by <15% in 1 M KOH at 80 °C for 7 days. These results indicated that polyethyleneimine is a promising raw material for the preparation of alkaline anion-exchange membranes used in fuel cell.

Preparation and properties of anion exchange membranes based on multiple quaternary ammonium groups

Anion exchange membrane fuel cell (AEMFC) has attracting great attention as one of the most clean energy conversion technologies, because it has a facile electrochemical kinetics under alkaline condition and enables the use of non-precious metal catalysts. The reported anion exchange membranes were usually prepared by tethering quaternary ammonium groups (QA) to the backbone of chloromethylated or bromomethylated aromatic polymers by immersing in trimethylamine solution. However, they exhibit much low ionic conductivity and poor alkaline stability, which hinders the further development of AEMFC technology. In order to improve the ionic conductivity of anion exchange membranes, we located multi QA groups on one side chains for enhancing the local density of ionic groups. Therefore, we synthesized a type of small molecule with a quaternary ammonium group and an alkyl bromide. After gethering it onto 1,4-diazabicyclo[2.2.2] octane-functionalized poly(arylene ether ketone) by a quaternization reaction, tri-quaternized poly(arylene ether ketone) (TQPAEK) was successfully prepared with flexible side chains containing multi quaternary ammonium groups. Then a series of anion exchange membranes in OH- form were obtained by solution casting and then alkaline treatment. The ionic exchange capacity of these membranes in the range of 1.75–2.57 meq/g could be adjusted by controlling the degree of bromination of the precursor polymer. The water uptake, swelling ratio and hydroxide conductivity of TQPAEK membranes increased with the increasing IEC values. They also showed good thermal and mechanical stabilities. Especially, the introduction of flexible side chains significantly improved the elongation at the break of membranes, which reached up to 103.2%. Meanwhile, the tensile stress was still higher than 28 MPa. AFM images showed that the hydrophilic/hydrophobic phase-separation morphology was formed in these TQPAEK containing multi-quaternized flexible side chains. Therefore, TQPAEK exhibited high ionic conductivity of 74.35 mS/cm at 80°C. The results indicated that TQPAEK membranes had the potential to be used in alkaline AEMFCs.

Preparation and properties of anion exchange membranes based on spirocyclic quaternary ammonium salts

<p indent=0mm>Among various types of fuel cells, alkaline anion exchange membrane fuel cells (AEMFCs) have attracted enormous attention as clean and highly efficient energy conversion devices for vehicles and portable electronic applications. As the key component of AEMFCs, anion exchange membranes (AEMs) act both as a barrier to separate the fuel and an electrolyte to transport hydroxide anion. To fulfill the application of AEMFCs, an AEM should possess good thermal stability, good mechanical properties, high conductivity and excellent alkaline stability. The lack of commercially available AEMs with excellent alkaline stability is limited the application of AEMs in the AEMFCs. In the recent years, a variety of AEMs based on poly(sulfone)s, poly(styrene)s, polypropylene, poly(arylene ether)s, poly(phenylene)s, poly(phenylene oxide)s and poly(olefin)s, have been synthesized for the application of AEMs. Most of these AEMs showed a similar chemical structure that a polymer backbone with with pendent quaternary ammonium cations. However, quaternary ammonium cations are unstable in alkaline environment due to the Hofmann degradation, the nucleophilic substitution and the ylide reaction, even in high temperature. Therefore, various types of AEMs based on some potential alternative cations, including imidazolium, guanidinium, phosphonium, metal-cation, pyridinium, tertiary sulfonium, and pyrrolidinium cations have been prepared and investigated in the last few years. Recently, different quaternary ammonium small molecules were synthesized and their alkaline stability was thoroughly investigated by Marino and Kreuer. Their study showed that spirocyclic quaternary ammonium have good stability in alkaline condition. To date, there is few work on spirocyclic quaternary ammonium-based AEMs, and to understand the properties of this type of AEMs, a systematic study is needed. In the present work, <italic>N</italic>,<italic>N</italic>-diallylpyrrolidinium bromide [DAPy][Br] was synthesized and used as hydrophilic phase in the polymeric membranes. The purity and chemical structure of [DAPy][Br] were confirmed by ¹H NMR measurements. However, poly(<italic>N</italic>,<italic>N</italic>-diallylpyrrolidinium bromide), the homopolymer of [DAPy][Br], shows poor film forming properties, and it is very soluble in water. Therefore, styrene and acrylonitrile were chosed to synthesize the copolymers with [DAPy][Br] due to the ease of processing, good mechanical properties and chemical resistance. A mixture of styrene/acrylonitrile, [DAPy][Br], divinylbenzene and benzoin ethyl ether was stirred to obtain a homogeneous solution. Then, the mixture was casted onto a glass mold irradiation with UV light. The prepared spirocyclic quaternary ammonium-based membranes were immersed in <sc>1 mol/L</sc> KOH solution at 60°C for <sc>24 h</sc> to convert the anion of membrane from Br^- to OH^-. Then the membranes was immersed in deionized water and washed with deionized water until the pH of residual water was neutral. In summary, a series of spirocyclic quaternary ammonium-based AEMs were successfully designed and prepared via a simple synthetic strategy. The resultant AEMs demonstrated great potential for alkaline anion exchange membrane fuel cell applications based on its good thermal stability, sufficient mechanical properties and high conductivity. The ion exchange capacity can be controlled by change the mixture ratio, and the swelling ratio, water uptake and ion exchange capacity of the spirocyclic quaternary ammonium-based membranes increase with the increasement of the content of [DAPy][Br] in the membranes. The transparent and mechanically robust spirocyclic quaternary ammonium-based AEMs show high conductivity up to <sc>7.29×10^-2 S/cm</sc> at 80°C, and all the membranes showed conductivity up to <sc>1×10^-2 S/cm</sc> at room temperature. In addition, all the AEMs synthesize in this work showed excellent long-term alkaline stability at elevated temperature. The spirocyclic quaternary ammonium-based AEMs with an aliphatic backbone exhibit high alkaline stability, which will open up new prospects for the preparation of AEMs with excellent alkaline stability and high conductivity.

Crosslinked poly (2,6-dimethyl-1,4-phenylene oxide) polyelectrolyte enhanced with poly (styrene-b-(ethylene-co-butylene)-b-styrene) for anion exchange membrane applications

Poly (styrene‑b‑(ethylene-co-butylene)‑b‑styrene) (SEBS) shows great potential in anion exchange membranes (AEMs) due to its high chemical stability and flexible mechanical properties, but the trade-off between the ion conductivity and gelation issues greatly impedes further applications. To address this dilemma, we present an effective strategy to enhance comprehensive properties of AEMs by fabricating a crosslinking structure between SEBS and poly (2,6-dimethyl-1,4-phenylene oxide) (PPO). Grafting multiple cations as the side chains ensures the highly efficiently transport of hydroxide. With the addition of SEBS, the crosslinked membranes (T3PPO-c-SEBS) show enhanced physical and chemical properties, such as restrained swelling behavior, firm mechanical properties, great chemical and dimensional stabilities. Obvious hydrophilic/hydrophobic microphase separation morphology is designed to form effective ionic channels for transmitting hydroxide. The T3PPO-c-30%SEBS membrane with 30 wt% content of SEBS exhibits relatively high hydroxide conductivity (24.0 mS/cm at 30 °C) but low swelling ratios (15.6% at 30 °C). Moreover, after the long-time alkaline test (1 M NaOH, 80 °C, 500 h) and antioxidative test (Fenton solution, room temperature, 150 h), the T3PPO-c-30%SEBS membrane shows much better alkaline stability (57.8% retention vs. 24.5%) and oxidative stability (92.0% retention vs. 85.8%) than the pristine T3PPO membrane does. Therefore, T3PPO-c-SEBS can be regarded as promising candidates for AEM applications.

Properties of anion exchange membrane based on polyamine: Effect of functionalized silica particles prepared by sol–gel method

Membranes of polyamine (PA-SiNH2)m, containing silica reacted with 3-aminopropyltriethoxysilane (APTES) in hydrolytic conditions were prepared via solution casting, followed by methylation and ion exchange process. The influence of amino-functionalized silica (Si-NH2) on the properties of the obtained membrane was investigated. Fourier transform infrared spectroscopy (FTIR) and Nuclear magnetic resonance spectroscopy (NMR) were used to investigate the chemical features of the silica and its interaction with the polyamine polymer. The results of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of the modified membrane confirmed it is stable up to 300 °C. The thermal stability is the result of the interaction of modified silica particles and polyamine polymer. It was demonstrated that the performance of the (PA-SiNH2)m anion exchange membrane is greatly improved by incorporation of silica nanoparticles as compared with the anion exchange membrane (PK-PDAPm), which doesn't contain silica. Therefore, the (PA-SiNH2)m is a suitable candidate for electrochemical applications.

Bent-twisted block copolymer anion exchange membrane with improved conductivity

Inadequate hydroxide conductivity of anion exchange membrane (AEM) is one of the obstacles restricting practical application of anion exchange membrane fuel cell. In this work, we report the design, fabrication and properties of AEMs based on bent-twisted non-coplanar block copolymer. The bent-twisted polymer backbone creates high free volume size so that hydroxide ion transport is facilitated. The free-volume effect can also facilitate microphase separation due to enhanced chain segment motion. With the free-volume holes working synergistically with microphase-separation enabled ion channels, the membrane showed a high conductivity (35mScm−1 at room temperature) at a relatively low ion exchange capacity (IEC, 1.25meqg−1); it also exhibited a good dimension stability, the swelling ratio being 4.7% at room temperature and kept virtually unchanged with temperature because of relatively low IEC. A H2/O2 fuel cell employing the fabricated AEM yielded a high power density (262mWcm−2 at 50°C). Our work opens up a new route to fabrication of highly conductive AEM with relatively low IEC.

Effects of hypochlorite exposure on the structure and electrochemical performance of ion exchange membranes in reverse electrodialysis

We performed chlorination experiments to understand the impact of hypochlorite exposure on both cation exchange membranes (CEMs) and anion exchange membranes (AEMs) regarding their properties and performance in a reverse electrodialysis (RED) stack. Changes in membrane morphology, surface elemental composition and chemical bounding suggested the chlorine incorporation in the form of C-Cl bonds and side-chain cleavage of –SO3- or –NR3+ containing molecular fractions. These observations were further supported by observed increases in hydrophobicity and decreases in fixed charged groups of the membranes, respectively. Compared to CEMs, AEMs were less chlorine resistant such that the development of more extensive cracks in the membrane structure further increased water content and dramatically decreased membrane conductivity. The performance of both chlorinated CEMs and AEMs were tested in an RED stack and the results showed the reduced RED power density was largely attributed to the deteriorated electrical properties of AEMs.

In-situ preparation of cross-linked hybrid anion exchange membrane of quaternized poly (styrene-b-isobutylene-b-styrene) covalently bonded with graphene

Introducing graphene into polymer matrix is an effective way to enhance performances of anion exchange membrane (AEM). However, utilizing the advantages of graphene by physical approach is limited due to the weak interface interaction between graphene and polymer matrix. Herein, we report an effective strategy to covalently bond graphene with polymer matrix to improve the interface interaction and further to improve the properties of AEM. A series of cross-linked quaternized graphene-based hybrid AEM were fabricated by covalently bonding poly (vinylbenzyl chloride) grafted graphene (GN-g-PVBC) copolymer with chloromethyl functionalized poly (styrene-b-isobutylene-b-styrene) (SIBS) through the cross-linker (N,N,N′,N′-tetramethyl-1,6-hexanediamine) by in-situ synthetic approach. The interface interaction between graphene and QSIBS is greatly enhanced according to micromorphology characterization of the hybrid membrane. The cross-linked quaternized hybrid AEM containing 0.55 wt% of GN-g-PVBC exhibits obviously improved dynamical mechanical properties (storage modulus: 418 MPa), ion conductivity (1.81 × 102 S cm−1), methanol barrier property (5.19 × 10−7 cm2 s−1), selectivity (3.49 × 104 S s cm−3) at 60 °C and especially a comparably excellent chemical stability to that of Nafion 115 due to the enhanced interface interaction between graphene and the polymer matrix.

Recyclable cross-linked anion exchange membrane for alkaline fuel cell application

Cross-linking can effectively solve the conductivity-swelling dilemma in anion exchange membranes (AEMs) but will generate solid wastes. To address this, we developed an AEM cross-linked via disulfide bonds, bearing quaternary ammonium groups, which can be easily recycled. The membrane (RC-QPPO) with IEC of 1.78 mmol g−1, when cross-linked, showed enhanced mechanical properties and good hydroxide conductivity (24.6 mS cm−1 at 30 °C). Even at higher IEC value (2.13 mmol g−1), it still has low water uptake, low swelling ratio and delivers a peak power density of 150 mW cm−2 at 65 °C. Exploiting the formation of disulfide bonds from -SH groups, the membrane can be readily cross-linked in alkaline condition and recycled by reversibly breaking disulfide bonds with dithiothreitol (DTT). The recycled membrane solution can be directly utilized to cast a brand-new AEM. By washing away the residual DTT with water and exposure to air, it can be cross-linked again and this process is repeatable. During the recycling and cross-linking processes, the membrane showed a slight IEC decrease of 5% due to functional group degradation. The strategy presented here is promising in enhancing AEM properties and reducing the impact of artificial polymers on the environment.

Poly(terphenylene) Anion Exchange Membranes: The Effect of Backbone Structure on Morphology and Membrane Property.

A new design concept for ion-conducting polymers in anion exchange membranes (AEMs) fuel cells is proposed based on structural studies and conformational analysis of polymers and their effect on the properties of AEMs. Thermally, chemically, and mechanically stable terphenyl-based polymers with pendant quaternary ammonium alkyl groups were synthesized to investigate the effect of varying the arrangement of the polymer backbone and cation-tethered alkyl chains. The results demonstrate that the microstructure and morphology of these polymeric membranes significantly influence ion conductivity and fuel cell performance. The results of this study provide new insights that will guide the molecular design of polymer electrolyte materials to improve fuel cell performance.

Biofouling phenomena on anion exchange membranes under the reverse electrodialysis process

Reverse electrodialysis (RED) is an electrochemical process for converting salinity gradient energy into electric energy. Biofouling of anion exchange membranes (AEMs) is a severe problem affecting RED performance. In this study, we examined the biofouling of AEMs during RED and attempted to improve the anti-biofouling properties of AEMs through surface modification. Biofouling was evaluated from the bacterial coverage percentage, obtained from surface analysis using scanning electron microscope (SEM) images of fouled AEM surfaces. The results showed that a polydopamine (PDA) coating improved the anti-biofouling properties of AEM under RED operation. Furthermore, in order to investigate the effect of electric current and electric field, RED was performed under open circuit conditions (i.e., only an electric field was generated) in addition to normal RED, which generated both an electric current and an electric field. Both the generated electric current and the generated electric field suppressed bacterial attachment on the AEM surface, thereby enhancing the anti-biofouling properties of AEMs along with the PDA modification. It was also found that electric current changed the shape of bacteria.

Adamantammonium as a novel functional group for anion exchange membranes with excellent comprehensive performances

In the pursuit of alkali stable cation as functional group for anion exchange membranes (AEMs), a novel adamantammonium (AdA) cation, which replacing methyl group of conventional trimethylammonium by an adamantyl group, was synthesized and investigated. 1H NMR spectroscopy was employed to quantify the extent of decomposition and the results showed that > 99% adamantammonium remaining after 7 days immersion in 2 M NaOH (in D2O) at 80 °C. The AdA functionalized benzyl type poly(2,6-dimethyl phenylene oxide) polyelectrolytes, PPO-AdAs, were further prepared and characterized the AEM properties. Not only the chemical durability of these new PPO-AdA membranes, interestingly, but the conductivity and dimensional stability (water uptake and in-plane swelling) are superior to the benzylmethylammonium-benchmark PPO-based AEM (PPO-TMA). With the similar IEC values, the water uptake of PPO-AdA-41 sample is much lower than PPO-TMA-28, however, the conductivity of the former (15.7 mS cm-1, chloride conductivity at 50 °C) is higher than the latter (13.3 mS cm-1). Overall, the results of this study offer a base-stable cation, adamantammonium, and the corresponding adamantammonium-based AEMs for fuel cell applications with the advantages of chemical stability, enhanced conductivity and excellent dimensional stability as well as facial functionalization technique.

Mimicking the cell membrane: bio-inspired simultaneous functions with monovalent anion selectivity and antifouling properties of anion exchange membrane.

A new bio-inspired method was applied in this study to simultaneously improve the monovalent anion selectivity and antifouling properties of anion exchange membranes (AEMs). Three-layer architecture was developed by deposition of polydopamine (PDA) and electro-deposition of N-O-sulfonic acid benzyl chitosan (NSBC). The innermost and outermost layers were PDA with different deposition time. The middle layer was prepared by NSBC. Fourier transform infrared spectroscopy and scanning electron microscopy confirmed that PDA and NSBC were successfully modified on the surfaces of AEMs. The contact angle of the membranes indicated an improved hydrophilicity of the modified membranes. A series of electrodialysis experiments in which Cl−/SO42− separation was studied, demonstrating the monovalent anion selectivity of the samples. The Cl−/SO42− permselectivity of the modified membranes can reach up to 2.20, higher than that of the commercial membrane (only 0.78) during 90 minutes in electrodialysis (ED). The increase value of the resistance of the membranes was also measured to evaluate the antifouling properties. Sodium dodecyl benzene sulfonate (SDBS) was used as the fouling material in the ED process and the membrane area resistance of modified membrane increase value of was only 0.08 Ωcm2 30 minutes later.

Effect of Bulky Cations on the Stability and Properties of Anion Exchange Membranes

Alkaline anion exchange membrane fuel cells are widely investigated recently as promising alternative to well-established proton exchange membrane fuel cells. Anion exchange membranes, using as solid electrolytes, are required to be thermally, chemically and mechanically stable in base at the operation temperature and humidity, as well as have an efficient OH- transport along the membrane across two electrodes. This study aims at studying AEMs functionalized with three different bulky cations. They are 1,4,5-trimethyl-2-(2,4,6-trimethoxyphenyl) imidazolium functionalized polyphenylene oxide (Figure 1(a)) [1], tris(2,4,6-trimethoxyphenyl) phosphonium functionalized PPO (Figure 1(b)) [2], and phenylene cobaltocenium functionalized norbornene (Figure 1(c)). Degradation study on 1,4,5-trimethyl-2-(2,4,6-trimethoxyphenyl) imidazolium functionalize PPO and unsubstituted imidazolium PPO by soaking both two membranes into 1 M sodium hydroxide at 80°C indicates attachment of 2,4,6-trimethoxyphenyl group efficiently improve OH- stability due to the enhanced volumetric steric and basicity of the central cation. Super hydrophobic and bulky phosphonium cation attached by three 2,4,6-trimethoxyphenyl groups has the chemical stability surpassing benzyl trimethyl ammonium by 30% after degrading in 1 M KOH at 80°C for 20 days. The cobaltocenium cation, which has 18 valance electrons further improve the chemical stability by 60% higher than trimethyl ammonium after treating in 1 M NaOH/CH3OH at 80°C for 20 days (Figure 2). Besides enhanced chemical properties from these bulky cations, their influence on the other polymer properties include thermal stability, water uptake, conductivity as well as transport property are also discussed. Figure 1. Structure of 1,4,5-trimethyl-2-(2,4,6-trimethoxyphenyl) imidazolium functionalized PPO (a), tris(2,4,6-trimethoxyphenyl) phosphonium functionalized PPO (b) and phenylene cobaltocenium functionalized norbornene (c). Figure 2. Degradation studies performed at 80°C on 1,4,5-trimethyl-2-(2,4,6-trimethoxyphenyl) imidazolium (solid diamond, 1 M KOH), tris(2,4,6-trimethoxyphenyl) phosphonium (solid square, 1 M KOH), phenylene cobaltocenium (solid triangle, 1 M NaOH/CH3OH) and benzyl trimethyl ammonium (open square, 1 M KOH; open triangle, 1 M NaOH/CH3OH). Reference [1] Y. Liu, J. Wang, Y. Yang, T.M. Brenner, S. Seifert, Y. Yan, M.W. Liberatore, A.M. Herring, J. Phys. Chem. C 118 (2014) 15136-15145. [2] Y. Liu, B. Zhang, C.L. Kinsinger, Y. Yang, S. Seifert, Y. Yan, C.M. Maupin, M.W. Liberatore, A.M. Herring, J. Membr. Sci. 506 (2016) 50-59. Figure 1

Effect of Chemical Structure on Properties of Multiblock Copolymers for Alkaline Membrane Fuel Cell

Alkaline membrane fuel cells (AMFCs), which use anion exchange membrane (AEM) as a solid electrolyte, continue to receive increased attention because of their advantages of fast oxygen reduction reaction in alkaline media, efficient water management, and the ability to use non-precious metal catalysts (e.g., nickel, cobalt, silver, iron, and etc.). Hydrocarbon-based aromatic polymers have been widely used as AEMs due to their good thermal and mechanical stabilities and easiness of synthesis and functionalization. However, poor chemical stabilities under strong alkaline environment and low ionic conductivity of AEMs should be improved for commercialization of AMFCs. Hydrophilic and hydrophobic phase separation is helpful for improvement in chemical stability and ionic conductivity. Well-developed hydrophilic domains form effective ion transport pathways resulting in enhanced ionic conductivity, while hydrophobic domains maintain mechanical and chemical stabilities because most nucleophilic sources, which can attack polymer backbone and cationic groups, are in hydrophilic domains. It is well known that block copolymers are the most promising chemical architecture for developing hydrophilic and hydrophobic phase separation. Based on these backgrounds, we synthesized poly(arylene ether sulfone) based block copolymers to investigate the effect of chemical structure on AEM properties. These block copolymers were functionalized with cations after selective chloromethylation of hydrophilic blocks. The degree of chloromethylation was controlled to optimize ion exchange capacity values of block copolymers. The molecular weight of each oligomer was controlled by changing the feed ratio of monomers and confirmed by nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC). All block copolymers showed well-developed morphology and outstanding AMFC performances. More detailed results will be discussed.

Effect of polydopamine coating and direct electric current application on anti-biofouling properties of anion exchange membranes in electrodialysis

The fouling of anion exchange membranes (AEMs) during an electrodialysis (ED) is a serious problem. Membrane fouling is often classified into organic fouling, inorganic fouling (scaling) and biofouling. In our previous studies, we reduced organic fouling by modifying the membrane surface with dopamine solution, which formed a negatively charged, hydrophilic polydopamine (PDA) layer on the membrane surface. PDA coating also increased monovalent anion permselectivity. Improvement in monovalent anion selectivity is expected to decrease scale formation and consequently improves anti-inorganic fouling properties. In this work, we investigated the effect of PDA coating on biofouling of AEM. The anti-adhesive properties were evaluated with a static adhesion test using Pseudomonas putida (P. putida). It was found that the anti-adhesive properties were effectively improved by surface modification with 0.1kg/m3 dopamine solution. Moreover, we evaluated biofouling during ED operation, for the first time, by measuring the time elapsed before biofouling occurred using a P. putida suspension as the feed solution. The surface was then observed by scanning electron microscope (SEM) after biofouling experiment. The results showed that a direct electric current application suppressed bacterial attachment on the AEM surface in addition to the effect of membrane surface modification with PDA.

Effect of Polymer Membrane Processing Techniques in Chemical and Mechanical Properties of Anion Exchange Membranes for Electrochemical Applications

Anion exchange membranes (AEMs) are promising materials for many electrochemical energy conversion and storage systems such as fuel cells, electrolysers, redox flow batteries, reverse electrolysers, just to name a few. We have observed that membrane-processing techniques affect mechanical and chemical properties of an anion exchange membrane, significantly. Literatures have shown highly conductive AEMs 1 at fuel cell operating conditions, however, a more study is still needed to find a balance between ionic conductivity and mechanical strength of AEMs for practical applications. We used poly(vinyl benzyl chloride)(PPO-b-PVBC) polymer with IEC of 2.6 mmol/g, synthesis published else where1, to understand the effect of polymer processing techniques in ionic conductivity and mechanical properties. Elemental analysis of the membrane after melt pressing for 5, 10 and 20 min of solvent cast membranes showed the decrease in chloride in the membrane by ~55, 44 and 44%; respectively from it’s initial 14.5 %w/w of chlorine in the polymer. The reduced chlorine but oxygen in the membrane indicated the presence of crosslinking in hydrophilic group of the polymer. Ionic conductivity, diffusion of ions and water, and dynamic vapor water sorption experiments were performed to understand ionic transport, tensile stretching of membrane were performed at fuel cell operating conditions to understand membrane strength, stiffness and elongation, and small angle x-ray scattering experiments were performed to understand the membrane morphology. Membranes with higher degree of cross-linking by melt pressed for 40 min, were found to be mechanically robust, but chemically less conductive. The membrane melt pressed for just 10 min, was highly conductive with slightly lower membrane strength. A different degree of cross-linking showed a difference in water uptake (Figure 1). A greater water uptake by membrane had bigger effect of water plasticizing the membrane resulting in poor mechanical performance. A good balance of degree of cross-linking in the membrane to optimize the ionic conductivity, and mechanical properties will be discussed in the presentation. Figure 1. Vapor water uptake by the membrane at 60°C as a function of humidity. SCMP5, SCMP10, and SCMP20 (solvent cast and then melt pressed for 5, 10 and 20 min; respectively in addition to waiting same amount of time for melting the polymer before the application of pressure) Acknowledgement The authors would like to thank the US Army Research Office for the funding under the MURI #W911NF-10-1-0520and Yating Yang and Daniel M. Knauss for providing the polymer for the study.

Mechanical properties of anion exchange membranes by combination of tensile stress-strain tests and dynamic mechanical analysis

The thermomechanical properties of anion exchange polymers based on polysulfone (PSU) quaternized with trimethylamine (TMA) or 1,4-diazabicyclo[2.2.2]octane (DABCO) and containing hydroxide or chloride anions by tensile stress–strain tests and dynamic mechanical analysis (DMA) have been determined. The reported mechanical properties included the Young's modulus, tensile strength, and elongation at break from tensile tests and the storage and loss modulus and glass transition temperature from DMA. The anion exchange membranes behaved as stiff polymers with Young's modulus in the order of 1 GPa, relatively with high strength (about 30 MPa) and low elongation at break (around 10%) was observed. Tensile tests were also made with membranes exchanged with hydrogen-carbonate and carbonate anions to control the absence of important carbonation of the OH form. The glass transition temperatures were of the order of 150 °C (PSU-TMA) or 200 °C (PSU-DABCO) for the hydroxide form, confirmed by differential scanning calorimetry; they increase further by about 50 K, when hydroxide ions are replaced by chloride. This result and the increase of the storage modulus could be interpreted by the higher hydration of hydroxide ions and the plasticizing effect of water, which reduced the Van der Waals interactions between the macromolecular chains. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1180–1187