Conductivity Of Anion Exchange Membranes Research Articles

Highly Conductive Anion-Exchange Membranes Based on Cross-Linked Poly(norbornene): Vinyl Addition Polymerization

Cross-linked (XL) anion-exchange membranes (AEMs) synthesized by vinyl addition polymerization of norbornene were prepared for use in anion-exchange membrane electrochemical devices, including fuel...

Enhancing hydroxide conductivity of anion exchange membrane via incorporating densely imidazolium functionalized graphene oxide

This study presents a generic method to increase the hydroxide conductivity of anion exchange membranes by tuning the microphase separation structure. Graphene oxide was functionalized with macromolecular brushes for the first time by a precipitation polymerization method. Densely-functionalized imidazolium groups were aligned in the configuration of macromolecular brushes to act as hydroxide-conductive groups, which endow the functionalized graphene oxide with a high ion exchange capacity value of 3.05 mmol g−1. Polymer-inorganic composite membrane for anion exchange membrane fuel cell was fabricated by incorporating the imidazolium-functionalized graphene oxide into imidazolium-functionalized bisphenol A-type polysulfone. The dense imidazolium groups manipulated the aggregation of conductive groups at the polymer/filler interfaces to induce the well-defined microphase structure of composite membranes, constructing low-resistance channels for ionic transport. The activation energy of hydroxide transport in composite membranes was reduced to 25.17–13.62 kJ mol−1, in comparison with 28.63 kJ mol−1 for control membrane. The hydroxide conductivity of composite membrane was elevated to 22.02 mS cm−1 at 30 °C, which is 2.10 times of that for control membrane. The maximum power density of single fuel cell of 78.7 mW cm−2 at 60 °C was thus achieved.

Predicting the Effects of Carbon Dioxide on the Conductivity of Electrospun Anion Exchange Membranes

Electrospinning is one processing technique that can be used to manufacture anion exchange membranes (AEMs) with good mechanical stability while also maintaining high conductivities. The unique properties of electrospun AEMs may affect their response to carbon dioxide absorption, a process known to reduce the conductivity of traditional AEMs. In this study, we develop two models that allow us to predict the response of these membranes upon exposure to CO2 in an operating fuel cell. In the first model, the morphological parameters of the electrospun membrane are analytically determined as a function of ionomer fiber content using a simplified picture of ion transport through the network of ionomer fibers. The results of this morphology model are then used in the second model, which numerically predicts the carbonation dynamics and ionic conductivity of the AEMs during fuel cell operation. The model predictions are validated to experiments performed on fuel cells employing well-known single-phase AEMs. The results herein investigate the membranes’ conductivity loss/reclamation as a function of operating current, cathode gas CO2 content, and ionomer fiber content.

Highly conductive anion exchange membranes based on one-step benzylation modification of poly(ether ether ketone)

Side-chain-type quaternized aromatic polyelectrolytes named QBz-PEEK-x have been synthesized via one-step benzylation modification of commercial poly(ether ether ketone) (PEEK). By controlling the molar ratio between the benzylation reagent and PEEK, the substitution degree x could be tuned in a wide range (51.1%~ 91.6%). A series of five anion exchange membranes (AEMs) with high mechanical strength were facilely prepared via solution-casting of QBz-PEEK-x. Water uptake, swelling ratio and ion conductivity of these AEMs were investigated in detail. Of particular interest is that, at 60 °C, the AEM of QBz-PEEK-76.0% exhibited a moderate water uptake of 56%, a low swelling ratio of 15%, and an exceptionally high OH– conductivity of 155 mS cm−1 which was comparable to the top values ever reported. Moreover, a promising peak power density of 391 mW cm−2 was achieved when assembling QBz-PEEK-76.0% AEM in a H2/O2 single cell operated at 70 °C. This unprecedented “benzylation” method opens a new door to novel high performance side-chain-type AEMs.

Alkali stability of anion exchange membrane

Anion exchange membrane fuel cells (AEMFCs) are attractive alternatives to proton exchange membrane fuel cells due to using non-noble metal catalysts and faster cathode reaction kinetics. Anion exchange membranes (AEMs) are one of the key materials composed of AEMFCs. The hydroxide conductivity of AEMs has been able to meet operating requirements of fuel cells. Although the hydroxide conductivity is no longer a problem, the stability of AEMs is still a notable challenge. This work mainly introduces the development of alkaline stability of AEMs. AEMs mainly consist of functional groups, side chains and polymer backbones. The degradation of functional groups causes by attacking of hydroxide ion through Hofmann elimination, nucleophilic substitution and ylide-intermediate degradation pathways. Numerous functional groups have been developed in order to improve the alkaline stability of AEMs. The introductions of appropriate electron donors and steric shielding are effective ways to weaken the attack of hydroxide ion. Developing new functional groups is a very important way. In addition, the development of novel stable groups is an important way for long-lived AEMs. For example, hetero-cycloaliphatic quaternary ammonium cations (QAs) are proved to be especially stable in both model compound studies and even AEM studies. The most commonly studied AEMs were synthesized by chloromethylation/bromination of the aromatic backbones, followed by a Menshutkin reaction to introduce QAs. Benzylic groups, electron-withdrawing groups, are prone to degrade by nucleophilic substitution on the benzylic carbon/α-carbon, Hofmann elimination and N-ylide intermediate formation in the presence of hydroxide ion. Therefore, the tethering linkage between the polymer backbone and functional groups is quite important. Recent studies have mainly introduced long side chains between functional groups and polymer backbones, leaving functional groups away from the benzene ring of the polymer backbone. The all-alkyl, amine- and ether-containing branched-chain structures are proved to be stable. Poly(arylene ether)s have been extensively studied in AEMs due to the advantages of good thermal stability, high mechanical property and easy modification. Frequently employed polyaromatic electrolytes are quaternized poly(aryl ether sulfone)s, which are prepared via the nucleophilic polycondensation reaction. The introduction of functional groups leads to degrade of these main chains containing aryl ether bonds, which are unable to avoid in the polycondensation reaction. It is necessary that main chains are devoid of aryl ether bonds so as to develop AEMs with excellent alkaline stability. Diels-Alder reaction, coupling reaction and acid-catalyzed Friedel-Craft polycondensation are effective methods for synthesizing aromatic backbones devoid of aryl ether bonds. In addition, aliphatic backbones, such as poly(ethylene-co-tetrafluoroethylene), are proved to have good stability in alkaline conditions, so its modification for AEMs has also become an important research direction. At present, the alkaline stability of AEMs has made great progress. Three major aspects have been studied in an effort to develop stable AEMs: (1) screening stable functional groups such as five- or six-membered cyclic amines, substituted imidazoles; (2) developing effective linkages between functional groups and polymer backbones; (3) designing aryl ether-free aromatic backbones and stable aliphatic backbones. The designed AEMs can remain for hundreds of hours without degradation in alkaline solution, which is a significant advancement in increasing alkaline stability of AEMs.

Highly conductive anion exchange membranes with low water uptake and performance evaluation in electrodialysis

The present work reports the preparation of a series of novel highly durable imidazolium-decorated anion exchange membranes (AEMs), with 3D network structure, via ultraviolet crosslinking reaction between 1-vinylimidazole and 1,6-hexanedithiol. AEM modified with trimethylamine (TMA) groups (no crosslinking, 45.7%) showed a significantly reduced water uptake within a range of 14.4–23.6% at 80 °C. Due to relatively good alkali-resistant of imidazolium groups and the compact structure stemmed from crosslinking network, the optimum cross-linked AEM (BPPO-Im 0.3) can retard the degradation and exhibits superior alkaline stability in 1.0 M NaOH at 60 °C for over 15 days, compared with TMA modified AEM. In ED application, BPPO-Im 0.3 AEM has a higher NaCl removal ratio of 77.82% than that of commercial AEM-Type II (74.13%) within 3 h experimental time. Accordingly, it shows higher current efficiency (67.43%) and lower energy consumption (1.94 kWh kg−1 NaCl), compared to commercial one (62.83%; 2.05 kWh kg−1). The facile fabrication process and the better-performance are suggestive of that BPPO-Im 0.3 is potentially applicable in ED.

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.

Thermodynamics of Counterion Release Is Critical for Anion Exchange Membrane Conductivity.

As the field of anion exchange membranes (AEMs) employs an increasing variety of cations, a critical understanding of cation properties must be obtained, especially as they relate to membrane ion conductivity. Here, to elucidate such properties, metal cation-based AEMs, featuring bis(norbornene) nickel, ruthenium, or cobalt complexes, were synthesized and characterized. In addition, isothermal titration calorimetry (ITC) was used to probe counterion exchange thermodynamics in order to understand previously reported differences in conductivity. The ion conductivity data reported here further demonstrated that nickel-complex cations had higher conductivity as compared to their ruthenium and cobalt counterparts. Surprisingly, bulk hydration number, ion concentration, ion exchange capacity, and activation energy were not sufficient to explain differences in conductivity, so the thermodynamics of metal cation-counterion association were explored using ITC. Specifically, for the nickel cation as compared to the other two metal-based cations, a larger thermodynamic driving force for chloride counterion release was observed, shown through a smaller Δ Htot for counterion exchange, which indicated weaker cation-counterion association. The use of ITC to study cation-counterion association was further exemplified by characterizing more traditional AEM cations, such as quaternary ammoniums and an imidazolium cation, which demonstrated small variances in their enthalpic response, but an overall Δ Htot similar to that of the nickel-based cation. The cation hydration, rather than its hydration shell or the bulk hydration of the membrane, likely played the key role in determining the strength of the initial cation-counterion pair. This report identifies for the first time how ITC can be used to experimentally determine thermodynamic quantities that are key parameters for understanding and predicting conductivity in AEMs.

Highly conductive anion exchange membranes with long flexible multication spacer

Poly(ether ketone)s bearing multication side chains are synthesized via nucleophilic polycondensation, demethylation and the Williamson reaction to prepare high performance anion exchange membranes (AEMs). The introduction of multication side chains is found to promote the formation of hydrophilic/hydrophobic phase segregated morphology, as confirmed by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). The TQAPEK-0.5 membrane with ionic exchange capacity (IEC) of 1.94 meq g−1 shows the highest hydroxide conductivity of 132.8 mS cm−1 at 80 °C. The first cationic group on the side chain is linked to the backbone via hexyleneoxy groups to render the AEM a good alkaline stability. As a result, TQAPEK-0.5 has only a 22.4% decrease in the conductivity after immersed in a 2 M aqueous KOH solution at 60 °C for 600 h. Besides, the peak power density of a single cell using the TQAPEK-0.5 membrane is observed to be 206 mW cm−2 at a current density of 400 mA cm−2. Furthermore, the as-prepared AEMs also exhibit good mechanical property and thermal stability, and hold promise for application in fuel cells.

Poly(arylene ether ketone) Copolymer Grafted with Amine Groups Containing a Long Alkyl Chain by Chloroacetylation for Improved Alkaline Stability and Conductivity of Anion Exchange Membrane

We report simple and controllable acetylation of poly(arylene ether ketone) (PK) followed by Menshutkin reaction (quaternization), to prepare a highly alkaline stable and conductive (phase-separated) anion exchange membrane (AEM). A long side alkyl chain grafted with ammonium groups facilitates decent hydrophilic/hydrophobic microphase separation. Prepared AEMs exhibited improved conductivity, alkaline stability, and water resistivity, because of well-designed membrane architecture. Under alkaline conditions, nucleophilic substitution reaction and/or Hofmann elimination reaction (β-hydrogen) are expected to protected by grafting of a large volumetric alkyl chain (C = 16) with amine groups responsible for steric hindrance. Suitably assessed PK-QD-48 AEM reveals 0.81 V open circuit voltage (OCV) corresponding to 25.53 mW cm–2 power density at 65 °C in alkaline direct methanol fuel cell (ADMFC) operation. The reported synthetic strategy offers a novel route for preparing highly conducting, and alkaline stabl...

A practical method for measuring the true hydroxide conductivity of anion exchange membranes

Hydroxide ions in anion exchange membranes (AEMs) are quickly exchanged for larger and less mobile anions (CO₃2− and HCO₃−) when the membrane is exposed to ambient air. Therefore, reported conductivity values of AEMs in hydroxide form are difficult to reproduce, and existing conductivity measurement techniques are not always reliable. Up to now, comparison of reported data for the hydroxide conductivity of different membranes has not been possible because tests have been performed not just with different anions, but also under different conditions and using different methods. In this work we present a practical and reproducible ex situ method for measuring the true value of the hydroxide conductivity of AEMs.

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.

Anion exchange membranes with branched ionic clusters for fuel cells

A highly conductive anion exchange membrane with branched ionic clusters exhibits an excellent fuel cell performance of 266 mW cm−2 at 60 °C.

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.

Impact of carbonation processes in anion exchange membrane fuel cells

Alkaline anion exchange membrane fuel cell (AEMFC) is a promising technology to replace precious metals used today as fuel cell catalysts. However, AEMFC does not yet demonstrate high performance when running on ambient air where they are exposed to CO2. The resulting carbonation reaction reduces membrane conductivity. This paper analyses and quantifies the effect of CO2 from ambient air on the concentration profiles in the membrane and the anode and, thus, assesses the CO2 impact on fuel cell performance. The physico-chemical model contains chemical and electrochemical reactions, liquid-gas phase equilibria as well as the transport processes in the cell. Results imply that a significant part of fed CO2 is absorbed in the cathode and is transported as carbonate ions to the anode. Concentration profiles in the membrane reveal an enrichment zone of CO2 in the membrane close to the anode, negligible HCO3− and a wide distribution of CO32− across the membrane. The carbonate distribution affects overall anion exchange membrane conductivity. For practical relevant current densities of i>500mAcm−2 and typical excess ratios of 1.5 for the hydrogen feed, less than 10% of the anions in the membrane are CO32−. We show that while increasing cell temperature has an ambiguous effect on the carbonation process and on the total effect of CO2 on the cell, current density has a significant effect. The impact of CO2 on AEMFC performance can be significantly decreased when operating the cell at high current densities above 1000mAcm−2.

Review of cell performance in anion exchange membrane fuel cells

Anion exchange membrane fuel cells (AEMFCs) have recently received increasing attention since in principle they allow for the use of non-precious metal catalysts, which dramatically reduces the cost per kilowatt of power in fuel cell devices. Until not long ago, the main barrier in the development of AEMFCs was the availability of highly conductive anion exchange membranes (AEMs); however, improvements on this front in the past decade show that newly developed AEMs have already reached high levels of conductivity, leading to satisfactory cell performance. In recent years, a growing number of research studies have reported AEMFC performance results. In the last three years, new records in performance were achieved. Most of the literature reporting cell performance is based on hydrogen-AEMFCs, although an increasing number of studies have also reported the use of fuels others than hydrogen - such as alcohols, non-alcohol C-based fuels, as well as N-based fuels. This article reviews the cell performance and performance stability achieved in AEMFCs through the years since the first reports in the early 2000s.

Improved conductivity and stability of anion exchange membrane modified with bi-phenylguanidinium bridged silsesquioxane

This work reports the design, fabrication and properties of 4,4′-oxydiphenylguanidinium-bridged-silsesquioxane (ODGBS) modified polysulfone anion exchange membrane (AEM). It was prepared by sol-gel polymerization of ODGBS in the network of cross-linked, quaternized polysulfone. With hydrophilic biphenylguanidinium and SiOSi moieties working synergistically, the ODGBS derived gel functions as a water “reservoir” in the membrane and thus gives rise to high water uptake, which facilitates hydroxide ion transport via a vehicular mechanism. High water uptake may also “dilute” hydroxide ion at the vicinity of cations and lessen the hydroxide attack on cations. Moreover, ODGBS does not contain β hydrogen atoms, thus reducing the likelihood of Hofmann elimination under hydroxide attack. Due to the above benefits, ODGBS modified AEM could exhibit an improved conductivity from 19 to 25 mS/cm2 at 30 °C, and a conductivity retention of 85% when treated in 1 M NaOH at 60 °C for 120 h, higher than that of the un-modified AEM (72%). This work provides a new strategy for enhancing stability and conductivity of AEM.

Poly tris (1-imidazolyl) benzene ionic liquids/Poly (2,6-dimethyl phenylene oxide) composite membranes for anion exchange membrane fuel cells

To improve the conductivity of anion exchange membranes without reducing the mechanical properties, a series of poly ionic liquids (PILs) with precursors of 1,3,5-tris(1-imidazolyl)benzene (TIB) containing tri-conduct groups were synthesized and incorporated into quaternized poly (2,6-dimethyl phenylene oxide) (QPPO) membranes. The new poly ionic liquids we designed contain ternary conduct groups in one monomer which are advantageous to form ionic channels and thus dramatically increase the conductivity of membranes. Various alkyl chains were introduced into TIB during polymerization to tune the distance between two tri-conduct groups. The resulting PILs/QPPO composite membranes showed good thermal stability, appropriate mechanical properties, enhanced conductivity and fuel cell performance. When the numbers of –CH2– between two tri-conduct groups reached 12, the composite membrane showed the highest conductivity and fuel cell performance (55.5 mS/cm at 80 °C and 168.9 mW/cm2). The composite membranes we prepared show potential application in anion exchange membrane fuel cells (AEMFCs).

Cationic Side-Chain Attachment to Poly(Phenylene Oxide) Backbones for Chemically Stable and Conductive Anion Exchange Membranes

A series of chemically stable and ionically conductive side-chain anion exchange membranes (AEMs) based on poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) backbones and alkyltrimethylammonium cations are reported in this work. Two alkyltrimethylammonium groups with n-propyl (C3) and n-pentyl (C5) alkyl chains were tethered onto PPO backbones through secondary amine moieties, resulting in two side-chain AEMs, that is, NC3Q-PPO and NC5Q-PPO and NC5 Q-PPO. In comparison to benzylic QA groups (e.g., benzyltrimethylammonium cations in quarternized PPO (QPPO) and benzylalkyldimethylammonium cations in comb-shaped PPOs (i.e., QC3-PPO and QC6-PPO)), the alkyltrimethylammonium cations of the side-chain PPOs, which do not possess highly reactive benzylic protons adjacent to both the aromatic ring and the cation, showed superior alkaline stability. After 30 days of aging in 1 mol/L NaOH solution at 80 °C, the retention of the conductivities of NC3Q-PPO (IEC = 2.17 mmol/g), NC5Q-PPO-40 (IEC = 2.03 mmol/g), and NC5Q-PPO-...

Crosslinked side-chain-type anion exchange membranes with enhanced conductivity and dimensional stability

Covalent crosslinking is considered to be a promising strategy to balance the dimensional stability and conductivity of anion exchange membranes (AEMs). Nevertheless, crosslinking using diamine crosslinkers would introduce hydrophobic alkyl chains into the ionic domain resulting in a significant decline in hydroxide conductivity and crosslinking via catalyst is discouraged. Herein, we presented a strategy for preparing crosslinked AEMs without using catalyst and the crosslinked moiety is away from the ionic domain. The crosslinking was carried out by converting the trifluorovinyl groups into perfluorocyclobutane groups via thermal treatment. The designed side-chain structure is responsible for the obvious hydrophilic/hydrophobic phase separated morphology and interconnected ion conducting channels, as confirmed by atomic force microscopy, transmission electron microscopy and small angle X-ray scattering. The obtained membrane exhibited high dimensional stability and a highest conductivity of 77.1mScm−1 at 80°C. Furthermore, the crosslinked AEMs also had robust mechanical properties, good thermal stability and reasonable alkaline stability.