Anion Exchange Membranes For Applications Research Articles

Hydroxide ion-conducting metal–organic frameworks for anion-exchange membrane applications

Metal–organic frameworks are promising materials for ion-exchange membrane applications. This review paper examines their current status and potential to meet the rigorous ion-conductivity requirements, while identifying promising approaches for improvement.

Macromolecular crosslink of imidazole functionalized poly(vinyl alcohol) and brominated poly(phenylene oxide) for anion exchange membrane with enhanced alkaline stability and ionic conductivity

Anion exchange membranes (AEMs) are important energy conversion device for fuel cell applications, where the overall redox reaction happened. Both alkaline stability and ionic conductivity should be considered in the long-term use of fuel cells. In this work, imidazole functionalized polyvinyl alcohol was designed as the functional macromolecular crosslinking agent to fabricate crosslinked AEMs with brominated poly(phenylene oxide) matrix. Benefitting from the macromolecular crosslinked structure, the membranes displayed enhanced ionic conductivity and alkaline stability at elevated temperature. Moreover, membrane with ion exchange capacity of 1.54 mmol/g displayed ionic conductivity of 78.8 mS/cm at 80 °C, and the conductivity could maintain 75% of the initial value after immersion in 1 M NaOH solution at 80 °C for 1000 h. Moreover, a peak power density of 105 mW/cm2 was achieved when the assembled single cell with c-91 was operated at 60 °C. These results indicated that the construction of macromolecular crosslinked AEMs have great potential in the practical application of anion exchange membranes fuel cells.

Iron and cobalt containing electrospun carbon nanofibre-based cathode catalysts for anion exchange membrane fuel cell

The use of Pt-based cathode catalyst materials hinders the widespread application of anion exchange membrane fuel cells (AEMFCs). Herein, we present a non-precious metal catalyst (NPMC) material based on pyrolysed Fe and Co co-doped electrospun carbon nanofibres (CNFs). The prepared materials are studied as oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts in alkaline and acidic environments. High activity towards the ORR in alkaline solution indicated the suitability of the prepared NPMCs for the application at the AEMFC cathode. In the AEMFC test, the membrane-electrode assembly bearing a cathode with the nanofibre-based catalyst prepared with the ionic liquid (IL) (Fe/Co/IL–CNF–800b) showed the maximum power density (Pmax) of 195 mW cm−2, which is 78% of the Pmax obtained with a commercial Pt/C cathode catalyst. Such high ORR electrocatalytic activity was attributed to the unique CNF structure, high micro-mesoporosity, different nature of nitrogen species and metal-Nx active centres.

Structure–Property Relationship in Poly(Terphenylene)-Based Anion Exchange Membranes; The Impact of Backbone Structure for the Free Volume Control in Alkaline Fuel Cell Application

The commercial application of anion exchange membrane fuel cells (AEMFCs) has been impeded by the lack of high ion conductivity and mechanical/chemical stability of the currently used anion exchange membranes (AEMs). A new class of aromatic polymers, which contain conducting head groups directly onto the polymer backbone were developed to attain long-term stability along with high conductivity. Inspired from this, a series of poly(terphenylene)-based anion exchange membranes devoid of benzylic sites or aryl-ether bonds, that are vulnerable to attack by hydroxide ion, were synthesized and were investigated for their physicochemical properties for alkaline fuel cell applications. The polymer backbones having N,N-dimethylpiperidinium conducting head group were synthesized using acid-catalyzed Friedel-Crafts polycondensation reaction between N-methyl-4-piperidone and meta/para-terphenyl monomers. Along with meta-terphenyl homopolymer, another three sets of copolymers were synthesized by introducing the kink structured meta-terphenyl in different ratios with para-terphenyl to control the free volume. The copolymer membrane, m-p-CoP-50, prepared from meta/para terphenyl monomers with a 1:1 molar ratio displayed a well-balanced water uptake, dimensional stability and the highest conductivity. The experimental procedures and the results will be discussed in detail.

Quaternary Ammonium-Bearing Perfluorinated Polymers for Anion Exchange Membrane Applications.

Perfluorinated polymers are widely used in polymer electrolyte membranes because of their excellent ion conductivity, which are attributed to the well-defined morphologies resulting from their extremely hydrophobic main-chains and flexible hydrophilic side-chains. Perfluorinated polymers containing quaternary ammonium groups were prepared from Nafion- and Aquivion-based sulfonyl fluoride precursors by the Menshutkin reaction to give anion exchange membranes. Perfluorinated polymers tend to exhibit poor solubility in organic solvents; however, clear polymer dispersions and transparent membranes were successfully prepared using N-methyl-2-pyrrolidone at high temperatures and pressures. Both perfluorinated polymer-based membranes exhibited distinct hydrophilic-hydrophobic phase-separated morphologies, resulting in high ion conductivity despite their low ion exchange capacities and limited water uptake properties. Moreover, it was found that the capacitive deionization performances and stabilities of the perfluorinated polymer membranes were superior to those of the commercial Fumatech membrane.

Mussel-inspired functionalized LDH as covalent crosslinkers for constructing micro-crosslinking fluorenyl-containing polysulfone-based composite anion exchange membranes with enhanced properties

The low ionic conductivity and poor dimensional stability severely influence the application of anion exchange membranes (AEMs). Therefore, how to balance the problem of excessive swelling of polymer matrix caused by increasing ion conductivity becomes an important challenge. Herein, 3-(dimethylamino)-1- propylamine (DMAPA) and polyethyleneimine (PEI) were grafted to the surface of layered double hydroxide (LDH) through mussel-inspired chemistry. The sheet-like functionalized LDH (DLDH and PLDH) as both filler and crosslinker was incorporated into the quaternized fluorenyl-containing polyethersulfone (QPFSU) matrix to get a series of new composite anion exchange membranes (AEMs). The as-prepared micro-crosslinked AEMs were found to form more hydrophilic conductive ion clusters owing to the generation of new quaternary ammonium groups during the crosslinking reaction. The micro-crosslinked AEMs have better dimensional stability while having higher OH − conductivity. Especially the ion conductivity of CQPFSU-1%-PLDH membrane can reach 86.8 mS cm −1 at 80 °C with excellent dimensional and chemical stability (SR is 9.6% at 80 °C, 21.8% decrease in OH − conductivity at 60 °C in 1.0 M NaOH for 200 h). Overall, the micro-crosslinking method for functionalized LDH modified AEMs achieves a balance between ion conductivity and dimensional stability of AEMs. • 2D sheet-shaped functionalized LDH was prepared by mussel-inspired chemistry. • Micro-crosslinked QPFSU-based AEMs were prepared using functionalized LDH as crosslinkers and fillers. • Micro-crosslinked AEMs exhibited high ion conductivity and good dimensional stability. • Micro-crosslinking method achieves a balance between ion conductivity and dimensional stability of AEMs.

Effect of side chain on the electrochemical performance of poly (ether ether ketone) based anion-exchange membrane: A molecular dynamics study

Ionic conductivity and alkaline stability are the key properties that limited the widespread application of anion-exchange membranes (AEM) in electrochemical energy conversion/storage systems. In recent years, quaternary ammonium functionalized poly (ether ether ketone) (PEEK) membranes serve as a promising solution due to the good mechanical and chemical properties. Varied ionic conductivity and alkaline stability could be obtained when the membrane contains different functionalized side chains. However, it's still a challenge to understand the mechanism from the experimental study because various parameters could affect the electro-chemical performance of the membranes. In this work, we conduct coarse-grained molecular dynamics simulations to investigate two PEEK-based membranes, in which the side chains contain one (SQ) or two (GQ) quaternary ammonium groups. The simulation results indicate the self-diffusion coefficients in SQ and GQ are quite similar which should not be the main reason for the improved ionic conductivity of GQ, while the obviously increased ion-exchange capacity of GQ should result in the improved ionic conductivity. Furthermore, the simulation reveals that more water molecules wrap around the OH− in GQ, which could lead to the improved alkaline stability in comparison to that of SQ. This work provides a deeper understanding for the design of grafted copolymer based AEM with QA functional side chains.

Blend anion exchange membranes containing polymer of intrinsic microporosity for fuel cell application

High hydroxide conductivity and alkali resistance of anion exchange membranes (AEMs) is the basic requirement for realizing the practical application of anion exchange membrane fuel cells. In this work, we develop a novel blend AEM incorporating polymer of intrinsic microporosity (PIM). PIM features highly rigid backbone and loose packing so that the resultant blend AEM containing PIM possesses increased free volume so that the ion clusters formed by microphase separation may get easily percolated and form pathways for hydroxide ion transport. The blend AEM containing 30% PIM shows a low ion exchange capacity (IEC) of 0.77 mmol g−1, but a relatively high conductivity of 16.9 mS cm−1 at 30 °C, and yields a H2/O2 fuel cell power density of 163 mW cm−2 at 60 °C. The membrane also shows a high mechanical robustness (tensile strength 43 MPa) and alkaline stability as compared with that without PIM incorporation. To the best of our knowledge, this work is the first report of PIM incorporation in blend AEM, and provides a new and effective strategy to promote hydroxide ion conduction in AEMs at relatively low IEC, which may pave the way to achieve better balanced conductivity and stability.

Branched poly(ether ether ketone) based anion exchange membrane for H2/O2 fuel cell

High-performance anion exchange membranes (AEMs) are in need for practical application of AEM fuel cells. Novel branched poly(ether ether ketone) (BPEEK) based AEMs were prepared by the copolymerization of phloroglucinol, methylhydroquinone and 4,4′-difluorobenzophenone and following functionalization. The effects of the branched polymer structures and functional groups on the membrane's properties were investigated. The swelling ratios of all the membranes were kept below 15% at room temperature and had good dimensional stability at elevated temperatures. The branching degree has almost no effect on the dimensional change, but plays a great role in tuning the nanophase separation structure. The cyclic ammonium functionalized membrane showed a lower conductivity but a much better stability than imidazolium one. The BPEEK-3-Pip-53 membrane with the branching degree of 3% and piperidine functionalization degree of 53% showed the best performances. The ionic conductivity was 43 mS cm−1 at 60 °C. The ionic conductivity in 1 M KOH at 60 °C after 336 h was 75% of its initial value (25% loss of conductivity), and the IEC was 83% of its initial value (17% loss of IEC), suggesting good alkaline stability. The peak energy density (60 °C) of the single H2/O2 fuel cell with BPEEK-3-Pip-53 membrane reached 133 mW cm−2 at 260 mA cm−2.

Anion Exchange Membrane Fuel Cell Application of Synthesized Poly(ether ether ketone) Containing Imidazolium

The proton exchange membrane fuel cells, which have been extensively studied at present, have excellent performance but have a disadvantage of using expensive platinum catalysts. On the other hand, anion exchange membrane fuel cells have the advantage of using non-precious metal catalysts, and anion exchange membranes are a key component. An important component, anion exchange membranes, must have high electrochemical and mechanical properties. For the application of anion exchange membranes, poly(ether ether ketone)(PEEK) based copolymers containing imidazolium groups were synthesized. The synthesized PEEK had different contents of imidazolium group. To prepare the free standing films, PEEK containing various equivalents of imidazolium groups was dissolved in n-methyl-2-pyrrolidone (NMP). The films were then prepared with various PEEK solutions through a Petri dish and then the properties of the films were evaluated. The ion exchange capacity was measured by back-titration method and the ion conductivity was calculated by membrane resistance method. The TGA and UTM were used to investigate the thermal and mechanical properties of the various PEEK films.

Improved Physicochemical Stability and High Ion Transportation of Poly(Arylene Ether Sulfone) Blocks Containing a Fluorinated Hydrophobic Part for Anion Exchange Membrane Applications.

A series of anion exchange membranes composed of partially fluorinated poly(arylene ether sulfone)s (PAESs) multiblock copolymers bearing quaternary ammonium groups were synthesized with controlled lengths of the hydrophilic precursor and hydrophobic oligomer via direct polycondensation. The chloromethylation and quaternization proceeded well by optimizing the reaction conditions to improve hydroxide conductivity and physical stability, and the fabricated membranes were very flexible and transparent. Atomic force microscope images of quaternized PAES (QN-PAES) membranes showed excellent hydrophilic/hydrophobic phase separation and distinct ion transition channels. An extended architecture of phase separation was observed by increasing the hydrophilic oligomer length, which resulted in significant improvements in the water uptake, ion exchange capacity, and hydroxide conductivity. Furthermore, the open circuit voltage (OCV) of QN-PAES X10Y23 and X10Y13 was found to be above 0.9 V, and the maximum power density of QN-PAES X10Y13 was 131.7 mW cm−2 at 60 °C under 100% RH.

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.

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.

Application of anion exchange membrane and the effect of its properties on asymmetric membrane capacitive deionization

Abstract Membrane capacitive deionization (MCDI) usually contains both anion and cation exchange membranes (AEM, CEM) to restrict the ion desorption during charging and re-adsorption during discharging in capacitive deionization (CDI). In this study, different from conventional MCDI, an asymmetric membrane capacitive deionization (AMCDI) device packing an AEM only (AMCDI-AEM) was constructed, where the AEM was lab-synthesized from poly (2, 6-dimethyl-1, 4-phenylene oxide). The effect of the AEM properties, such as ion exchange capacity, water uptake and membrane resistance on the desalination performance of AMCDI-AEM was systematically discussed. The results indicate that an AEM with high ion exchange capacity, low membrane resistance and low water uptake is beneficial for AMCDI-AEM. Furthermore, AMCDI packed with a commercial cation exchange membrane only (AMCDI-CEM) and a conventional MCDI device were also assembled to stress the importance of AEM application. Although the charge efficiency of AMCDI-AEM is lower than a full MCDI cell (54.7 vs 95.0%) due to the unprotected cathode, AMCDI-AEM device shows comparable salt adsorption capacity to MCDI (7.4 vs 7.2 mg g−1), and is much better than that of CDI (2.3 mg g−1) and AMCDI-CEM (1.5 mg g−1), suggesting that AEM plays a stronger role than CEM for membrane-assisted CDI application. By using a single AEM and commercial activated carbon electrode, this work provides an opportunity to reduce membrane cost for the industrialization of MCDI technology.

Crosslinked high-performance anion exchange membranes based on poly(styrene-b-(ethylene-co-butylene)-b-styrene)

Development of anion exchange membranes with good dimension, chemical stability, high conductivity and long life-time simultaneously is still a challenge for practical application of anion exchange membrane fuel cells. Herein, a mechanically and chemically stable poly(styrene-b-(ethylene-co-butylene)-b-styrene) triblock copolymer based crosslinked membranes are designed, prepared and characterized. N,N,N′,N′-tetramethyl-1,6-hexanediamine is introduced into the membranes as the crosslinker and functional reagent, which exhibits the significant improvement in water uptake (less than 7%), swelling ratio (8–5%, greatly smaller than uncrosslinked membranes) and tensile strength (higher than 18MPa). The hydroxide conductivity (14.9mScm−1 at 30°C) of the obtained membrane is obtained due to hydrophilic/hydrophobic microphase separation formed between the structure of crosslinking and quaternary ammonium groups. It demonstrates that incorporating stable crosslinking structure to polymer backbone greatly improves the chemical stability under Fenton and alkaline long-term tests. Furthermore, the peak power density of an H2/O2 single fuel cells reaches high peak power density of 416mWcm−2 at 0.514V. Based on these outstanding properties, the developed crosslinked membranes will be a promising candidate material for anion exchange membrane fuel cells.

A hamburger-structure imidazolium-modified silica/polyphenyl ether composite membrane with enhancing comprehensive performance for anion exchange membrane applications

With the intention of improving the comprehensive performance of anion exchange membranes (AEMs), a series of hamburger-structure AEMs based on triple-ammonium side chain poly(2,6-dimethyl-1,4-phenylene oxide) (TA-PPO) membrane were prepared. 1, 2-dimetylimidazolium-modified silica (Im-SiO2) was introducing to the surface of the TA-PPO membrane to fabricate a hamburger structure of Im-SiO2/TA-PPO membrane. The hamburger structure of the Im-SiO2/TA-PPO membrane is clearly observed by scanning electron microscope (SEM) and energy dispersive X-ray (EDX) linear scanning. Surprisingly, the Im-SiO2 layer on the surface of TA-PPO shows many valuable properties. Such as impressive ion conduction ability, water retaining capacity, and swelling resistance. Besides, the Im-SiO2 layer serves as a preserver to protect the TA-PPO membrane from being attacked by hydroxide and oxygen radical. Compared with TA-PPO membrane, the Im-SiO2/TA-PPO composite membrane exhibits the higher hydroxide conductivity and dimensional stability, and even higher alkaline stability and oxidation resistance. Typically, the Im-SiO2/TA-PPO composite membrane exhibits a maximum hydroxide conductivity of 105 mS/cm at 80 °C with a swelling ratio of 8.2%. Therefore, the hamburger structure of the Im-SiO2/TA-PPO membranes provides a new and reliable method to improve the comprehensive performance of AEM.

Magnetic field-oriented ferroferric oxide/poly(2,6-dimethyl-1,4-phenylene oxide) hybrid membranes for anion exchange membrane applications.

Concentrating on the ion conductivity of anion exchange membranes (AEMs), we present a magnetic-field-oriented strategy to address the insufficient ion conductivity and the lifetime problem of AEMs used in alkali membrane fuel cells (AMFCs). Magnetic ferroferric oxide (Fe3O4) is functionalized with quaternary ammonium (QA) groups to endow the QA-Fe3O4 with ion-exchange ability. A series of aligned QA-Fe3O4/poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) hybrid membranes were fabricated by doping QA-Fe3O4 in a triple-ammonium-functionalized PPO (TA-PPO) solution in an applied magnetic field. The structure of aligned QA-Fe3O4 in the TA-PPO membrane is clearly observed by using a scanning electron microscope (SEM). More importantly, the aligned QA-Fe3O4 constructs successive and effective ion-transport channels in the QA-Fe3O4/TA-PPO membrane, which dramatically improves the ion conductivity of the membranes. Notably, the magnetic-field-induced ion channels (MICs) are different from microscopic phase-induced ion channels (PICs). These MICs display much shorter ion transport distances and broader water channels than traditional PICs in AEMs. The aligned QA-Fe3O4/TA-PPO hybrid membrane displays a further 55% increase in ion conductivity after magnetic-field orientation compared to the normal QA-Fe3O4/TA-PPO membrane. Surprisingly, the aligned QA-Fe3O4 also improves the alkali stability and fuel cell performance of the hybrid membrane. The aligned 6%-QA-Fe3O4/TA-PPO hybrid membrane realizes a maximal power density of 224 mW cm-2. In summary, this work provides a novel and effective method to prepare high-performance AEMs.

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.

Anion Exchange Membrane Ionic Conductivity in the Presence of Carbon Dioxide under Fuel Cell Operating Conditions

A main obstacle in the widespread application of anion exchange membranes (AEMs) and AEM fuel cells (AEMFCs), aside from high-pH stability issues, is the low conductivity of the membrane [1]. Despite advantages such as more facile oxygen reduction reaction kinetics [2], even high performing AEMFCs still exhibit power densities many times lower than their proton exchange counterparts [1-3]. Because AEM is inherently less conductive, it is important that their conductivity behavior is well understood in order to mitigate any factors that result in further decrease in the membrane’s conductivity, or at least design around them. Furthermore, potential applications for AEMs and AEMFCs involve exposing the AEM to carbon dioxide [2, 4, 5], which reacts to form carbonate and bicarbonate species and leads to a significant decrease in the membrane’s conductivity [1-7]. Any devices intending to operate an AEM in the carbonate state would benefit from a detailed understanding of these ions’ effect on membrane performance. Efforts to elucidate the conductivity behavior of AEMs have been mostly experimental, but some theoretical models have also been developed using various approximations [5, 10-13]. Some past models assume chemical equilibrium among ionic species, however in an operating AEM this may not be the case. Furthermore, most of the theoretical models describe the behavior of isolated AEMs, that is, in the absence of polarization and electrochemical reactions that supply and consume ions and influences the ion exchange process. This work will incorporate these effects in order to predict the conductivity of an operating AEM in the presence of CO2 and under various operational current densities and temperatures. The model itself is a direct extension of [7], in that it adapts the same reaction mechanism for CO2 conversion to carbonate/bicarbonate ions in the AEM. Additional fundamental insight into the self-purging mechanism and potential scenarios for the recovery of membrane conductivity will be discussed. Acknowledgements Financial support from the Army Research Office (award number W911NF-14-1-0298) is gratefully acknowledged.