Similar Ion Exchange Capacity Research Articles

Simulated and Experimental Trends Regarding Water Uptake in Polymeric Electrolyte Membranes.

Polymeric membranes in an anion or a proton exchange membrane fuel cell need sufficient hydration in order to provide a high hydroxide ion or proton conductivity. The water uptake for six model ionomer membranes, all of the same ion exchange capacity, is modeled by dissipative particle dynamics. The architectures cover three types of families that are of potential interest in fuel cell membrane research. All architectures consist of connected hydrophobic backbone A beads, to which side chains are grafted. For the type I family, the hydrophilic (functional) C beads are pendent on (amphiphilic) [AxC] side chains. The type II architecture contains both hydrophobic [A4] and short hydrophilic [C] side chains. For type III, the C beads are embedded along various locations within the [AxCAy] side chains (x + y = constant). For similar equilibrium time, the membrane water volume fraction increases with side chain length x for type I, and for type III, it increases with the distance x that C beads are separated from the backbone. Among the architectures (types I and III) for which the number of covalent C-A bonds are the same, the water uptake increases with the average number of A-A and A-C bonds (dpd springs) between A beads and the nearest C bead. A picture emerges in which for similar ion exchange capacity model membranes water uptake increases as a function of ⟨Nbondphob-phyl⟩.

Controlling the microphase morphology and performance of cross-linked highly sulfonated polyimide membranes by varying the molecular structure and volume of the hydrophobic cross-linkable diamine monomers

Morphological control is essential for tuning the performance of polymer electrolyte membranes (PEMs). Block copolymerization is an effective but complicated way to control the morphology of PEMs. It is urgent to explore a simple alternative method to achieve this target. Herein, the molecular engineering strategy that changes the molecular structure and volume of hydrophobic cross-linkable diamine and introduces a hydrophobic network structure was used to optimize the microphase morphology of sulfonated polyimides (SPIs). Three diamine monomers containing tetrafluorostyrol pendant groups have been synthesized (4-(2,3,5,6-tetrafluoro-4-vinylphenoxy)benzene-1,3-diamine (TFVPDM), 1,3-bis(2-trifluoromethyl-4-amino-phenoxy)-5-(2,3,5,6-tetrafluoro-4-vinylphenoxy)benzene (6FTFPB), 2,2-bis(3-amino-4-(2,3,5,6-tetrafluoro-4-vinylphenoxy)phenyl)hexafluoropropane (6FATFVP)). Based on these monomers, 4,4′-diaminostilbene-2,2′-disulfonic acid (DASDSA) and 1,4,5,8-naphthalenetetracarboxylicdianhydride (NTDA), three cross-linked SPIs (CSPIs: CSPI-T-10, CSPI-TP-10, CSPI-AP-10) with similar ion-exchange capacity (IEC) value of 2.29–2.46 meq. g−1 but different main-chain structures were prepared through polycondensation and thermal cross-linking process. The introduced hydrophobic cross-linked structure improves the oxidative stability and methanol resistance of CSPI membranes. The CSPI membranes prepared from cross-linkable hydrophobic diamine with larger molecular volume and containing hydrophobic structural units in the main chain exhibit more pronounced phase separation morphology and higher proton conductivity. Moreover, the methanol resistance of CSPI membranes is dependent on the combined effect of IEC, degree of crosslinking (DC), and main-chain structure. The higher DC and main-chain containing hydrophobic structure are beneficial for enhancing the methanol resistance. The CSPI-AP-10, which was prepared from the 6FATFVP with the largest molecular volume, more hydrophobic cross-linkable pendant groups and hydrophobic linkages, exhibits the best performance. The CSPI-AP-10 membranes showed high proton conductivity and excellent methanol resistance. The direct methanol fuel cells (DMFCs) assembled from CSPI-AP-10 have a high max power density (PDmax) of 106.2 mW cm−2 in 2 M methanol and 75.1 mW cm−2 in 10 M methanol. The results indicated that the CSPI-AP-10 was a promising candidate as a PEM in DMFC application.

Fuel cell performance improvement via the steric effect of a hydrocarbon-based binder for cathode in proton exchange membrane fuel cells

In this study, a sulfonated poly(ether sulfone) having cardo-type fluorenyl groups (FL-SPES) was investigated as a cathodic binder to improve fuel cell performance via increased the oxygen diffusion in the cathode. The maximum power density achieved by using the membrane electrode assembly (MEA) prepared with FL-SPES with a low ion exchange capacity (IEC) of 1.31 meq g–1 was 520 mW cm–2, which is more than twice as high as that of BP-SPES (210 mW cm–2) having typical biphenyl groups with a similar IEC. At high IEC of 1.55 meq g–1, the power density obtained by using BP-SPES was improved to 454 mW cm–2 but remained lower than that of FL-SPES. In addition, although the IEC, swelling degree, and specific resistance were similar to each other, the gas permeability of FL-SPES was improved by approximately three times compared to that of BP-SPES. The steric structure of cardo-type FL-SPES increased the free volume between the polymer backbones, leading to an increase in gas transfer. Consequently, oxygen diffusion was promoted at the cathode, resulting in improved fuel cell performance.

The Influence of Pre-Irradiation and Simultaneous Grafting Methods on the Physicochemical Properties of Polyethylene-Based Anion-Exchange Membranes and Ionomers

Controlling the synthesis parameters during radiation-induced grafting (RIG) is a key feature to design anion-exchange membranes (AEMs) with enhanced properties that lead to high performances and stability. The present study provides a systematic investigation of the effect of RIG methods on physicochemical properties of LDPE-based AEMs with similar degree of grafting (DoG) and ion exchange capacity (IEC). The LDPE films were grafted with 4-vinylbenzyl chloride using two methods: i) pre-irradiation (PIM), using electron-beam (EB), and ii) simultaneous (SM), using gamma-ray (γ), both in air atmosphere and with different absorbed doses. The AEMs grafted by SM were irradiated with 20, 25, and 30 kGy, while the ones grafted by PIM were irradiated with doses of 30, 70, and 100 kGy. Samples grafted by the two different methods have the same molecular structure, but distinct physicochemical properties due to markedly differences in the degree of crosslinking. The studies have shown that grafting step directly influences the internal structure and morphology of AEMs. Multi-technique characterization demonstrated that RIG method determines the mechanical properties, water transport, and the distribution of ionic groups in AEMs. These characteristics have direct impact on fuel cell performance, anion-conductivity, and AEM stability.Figure 1 shows the polarization curves of anion-exchange membrane fuel cells (AEMFCs) operating at 80 °C with AEMs grafted by PIM and SM. Comparing AEMFCs performances containing AEMs with similar IECs from the two different grafting methods, it is possible to notice that AEMs synthesized by PIM, in general, promote better fuel cell performances than the corresponding ones prepared by SM. The main differences in the polarization curves are evidenced in the ohmic and the mass transport limiting regions. This result suggests that water management is enhanced in AEMs synthesized by PIM compared to the SM-AEMs. A membrane that allows rapid back diffusion of liquid water from the anode to the cathode, should reduce the current losses due to mass transport limitations.The gel content results confirmed the presence of more crosslinking in membranes synthesized by PIM than in the ones synthesized by the SM. As a consequence of that, enhanced mechanical properties were found for PIM-AEMs, indicating polymer backbone reinforcement due to high degree of crosslinking. Small-angle X-ray scattering measurements have confirmed that the microstructure of PIM and SM AEMs are different and this feature is directly related to ion conduction and water transportation.Figure 1. AEMFCs performances at 80 °C with LDPE-based AEMs grafted by the pre-irradiation method and by the simultaneous method. H2 anode gas flow=0.8 L min-1, O2 cathode gas flow=0.5 L min-1, both supplied unpressurized with optimal dewpoint temperatures ranging from 77 to 80 °C. AEMs are referred according to the grafting method and absorbed dose. 30 and 70-EB-PIM refer to the samples irradiated with 30 and 70 kGy, respectively, in the EB accelerator and grafted by the pre-irradiation method. 20 and 25-γ-SM refer to AEMs irradiated with 20 and 25 kGy, respectively, by γ-rays and grafted by the simultaneous method. Figure 1

Stable alkoxy chain enhanced anion exchange membrane and its fuel cell

In order to investigate the effect of side-chain position and polarity on the performance of anion exchange membrane (AEM), we design and fabricate a series of AEMs with alkyl extender (i.e. the cation's “tail”), alkyl additional side-chain (i.e. the cation's “neighbor”), alkoxy extender, alkoxy additional side-chain, and side-chain-free structure, which are named MImPSf-EC (EC = alkyl extender), MImPSf-AC (AC = alkyl additional side-chain), MImPSf-EO (EO = alkoxy extender), MImPSf-AO (AO = alkoxy additional side-chain) and MImPSf (side-chain-free), respectively. Our results indicate that both additional side-chain and alkoxy chain can promote micro-phase separation, and improve water uptake and conductivity of AEMs, which follow the order of MImPSf-AO > MImPSf-EO > MImPSf-AC > MImPSf > MImPSf-EC at similar ion exchange capacity (IEC). The AEM containing additional alkoxy chain, i.e. MImPSf-AO-150% exhibits the highest water uptake (77.6%) and hydroxide conductivity (23.7 mS/cm) at 30 °C. Compared with others, the alkoxy-containing AEM gives rise to higher H 2 /O 2 fuel cell peak power density (404 mW/cm 2 for MImPSf-AO and 383 mW/cm 2 for MImPSf-EO) and longer durability (137 min for MImPSf-AO and 133 min for MImPSf-EO at 100 mA/cm 2 ) at similar IEC due to its good hydrogen bond network and high stability of alkoxyl chain (both experimentally and theoretically confirmed). Our work demonstrates the advantage of alkoxy chain in AEMs as a partial replacement of degraded cationic groups so that the integrity of ion transport channel can be maintained, and fuel cell durability can be improved. • The effect of side-chain position and polarity on AEM performance was investigated. • The AEMFC employing alkoxy-containing AEMs yielded power density of 404 mW/cm 2 due to the good hydrogen bond network. • Stable alkoxy chain in AEMs enhanced the integrity of ion transport channel, thus improved fuel cell durability.

Complementary side chain promotes microphase separation in the membranes for alkali fuel cells

Anion exchange membranes (AEMs) play a crucial role in alkali electrolyte membrane fuel cells. Side chain grafted AEMs can undergo microphase separation and form good pathways to promote hydroxide ion transport; such AEMs typically have uniform side chain length at all repeat units of backbone, which may suffer from steric hindrance during cation aggregation and thus lead to insufficient microphase separation. In this work, we propose a complementary side chain structure, where a small amount of short side chain (C1) quaternized poly (dimethyl phenylene oxide) (qPPO-C1) is blended with long side chain AEM (qPPO-C6). Our results suggest that a blend AEM containing 5% qPPO-C1, whose IEC (ion exchange capacity) is as low as 1.45 mmol/g, exhibits an impressive conductivity of 37.5 mS/cm (30 °C) while its counterpart AEM with purely long side chains (qPPO-C6) displays an inferior conductivity (31.4 mS/cm) at a similar IEC. Its hydrogen/oxygen fuel cell yields a peak power density of 192 mW/cm2 when operating at 60 °C. Our work discloses, for the first time to our best knowledge, the advantage of complementary side chain in promoting microphase separation, in particular, at low IECs; it may offer new opportunities for further improvement of AEM performance.

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

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

Polynorbornene-based anion exchange membranes with hydrophobic large steric hindrance arylene substituent

To restrain excessive water uptake (WU) and swelling ratio (SR) of the quaternized polynorbornene anion exchange membrane (AEM) without sacrificing its anion conductivity, hydrophobic arylene substituent was introduced into polymer chain. The resultant cyclic olefin polymers (PBH-x-QA series) exhibited high glass-transition temperature (Tg) and tensile strength to meet the most important requirements of use as AEMs. Besides, the PBH-x-QA with ion exchange capacity (IEC) value ranged from 1.26 to 2.08 meq/g showed appropriate water uptake, good dimensional stability, and excellent tensile properties. The PBH-50%-QA exhibited water uptake of 31.0 wt% and swelling ratio (SR) of 17.6%. These values were much lower than those of PBN-26%-QA (WU = 95.5%, SR = 33.8%), which exhibited similar IEC value but had no arylene substituent. Notably, the PBH-60%-QA showed excellent hydroxide conductivity up to 101.9 mS/cm at 80 °C without excessive swelling in water, while PBH-50%-QA membrane showed excellent alkaline stability without visible loss of hydroxide conductivity, even after being immersed in 1 M NaOH aqueous solution over the 16-day test at 80 °C. A platinum-catalyzed H2/O2 fuel cell with PBH-50%-QA AEM presented a peak power density up to 228 mW cm−2 without optimized membrane electrode assembly fabrication process.

Highly Proton-Conducting Membranes Based on Poly(arylene ether)s with Densely Sulfonated and Partially Fluorinated Multiphenyl for Fuel Cell Applications.

Series of partially fluorinated sulfonated poly(arylene ether)s were synthesized through nucleophilic substitution polycondensation from three types of diols and superhydrophobic tetra-trifluoromethyl-substituted difluoro monomers with postsulfonation to obtain densely sulfonated ionomers. The membranes had similar ion exchange capacities of 2.92 ± 0.20 mmol g−1 and favorable mechanical properties (Young’s moduli of 1.60–1.83 GPa). The membranes exhibited considerable dimensional stability (43.1–122.3% change in area and 42.1–61.5% change in thickness at 80 °C) and oxidative stability (~55.5%). The proton conductivity of the membranes, higher (174.3–301.8 mS cm−1) than that of Nafion 211 (123.8 mS cm−1), was the percent conducting volume corresponding to the water uptake. The membranes were observed to comprise isolated to tailed ionic clusters of size 15–45 nm and 3–8 nm, respectively, in transmission electron microscopy images. A fuel cell containing one such material exhibited high single-cell performance—a maximum power density of 1.32 W cm2 and current density of >1600 mA cm−2 at 0.6 V. The results indicate that the material is a candidate for proton exchange membranes in fuel cell applications.

Architecture dependent water uptake in model polyelectrolyte membranes

The effect of polymeric architecture on the water uptake in ionomer membranes is modelled by Dissipative Particle Dynamics. We consider four polymers (a-d) of similar ion exchange capacity that contain hydrophobic A and hydrophilic C beads. For architecture a (A[A]C[A][A2])5 and b (A[A]C[A3])5 the C beads are located along the backbone that contain respectively 3 (a) and 2 (b) hydrophobic side chains per repeat unit. For architectures c ((A[C]A4])5) and d ((A[C]A[A3])5) each C bead represents a short hydrophilic side chain. Water is modelled by W beads. For ~20 nm thick membranes the water uptake is followed. The equilibrium water volume fraction varies between ~0.075 (architecture a) and ~0.4 (architecture d), and increases with (1) decreasing amount of C-A bonds per repeat unit and (2) the average number of bonds between hydrophobic beads and the nearest C bead, <Nbondphob-phil>. The potential energy per bead at equilibrium decreases as function of <Nbondphob-phil>. Water uptake is anticipated to show an increasing trend with increase of <Nbondphob-phil> for various architecture families. Some supporting evidence from experiments in literature is discussed. These results are of relevance for the optimization of proton and hydroxide ion conducting pores in electrochemical devices.

Synthesis and Characterization of 4‐Vinylimidazolium/Styrene‐Cografted Anion‐Conducting Electrolyte Membranes

AbstractThe β‐hydrogen‐free imidazole monomer, 2‐methyl‐N‐methyl‐4(5)‐vinylimidazole (2M4VIm), is synthesized to prepare anion exchange membranes (AEMs). The radiation‐induced graft polymerization of 2M4VIm and styrene is performed in poly(ethylene‐co‐tetrafluoroethylene), followed by N‐alkylation and ion exchange reactions to prepare 4‐vinylimidazolium‐containing AEM (2M4VIm/St‐AEM). AEMs that have a 6/4 2M4VIm/St molar ratio and 1.7 mmol g−1 ion exchange capacity (IEC) result in 75 mS cm−1 conductivity and 60% water uptake at room temperature in water, demonstrating enhanced values compared to previously reported 2‐methyl‐N‐vinylimidazolium‐containing AEMs (2MNVIm/St‐AEM) that have a similar comonomer ratio and IEC. The small‐angle neutron scattering measurements of dry and swollen 2M4VIm/St‐AEM reveal hydrophilic/hydrophobic two‐phase separation, as observed for 2MNVIm/St‐AEM with a similar Im/St composition. 2M4VIm/St‐AEMs exhibit higher alkaline stability in 1 m KOH at 80 °C at an early stage owing to the suppression of imidazolium β‐elimination but lower long‐term stability than that of 2MNVIm/St‐AEM due to the ring‐opening reaction of the imidazolium group due to its outward orientation from the graft main chain.

Ion exchange capacity controlled biphenol-based sulfonated poly(arylene ether sulfone) for polymer electrolyte membrane water electrolyzers: Comparison of random and multi-block copolymers

Conventionally, highly proton conductive perfluorosulfonic acid (PFSA) ionomers are used for an electrolyte membrane in a polymer electrolyte membrane water electrolyzer. As alternatives to the expensive and highly hydrogen permeable PFSA membranes, hydrocarbon-based sulfonated poly(arylene ether sulfone) proton conducting polymers (BPSH) are synthesized by varying their ion exchange capacity (IEC) from 1.2 to 2.0 meq/g, and the way to employ ion conducting units into polymers (random versus block). BPSH membranes show much lower hydrogen permeability (20 − 45 barrer) than that of PFSA membranes (~115 barrer for Nafion) at 80 °C and 100%RH. Random BPSH membranes show slightly higher selectivity of proton to hydrogen than the membranes from multi-block BPSH copolymers at a similar IEC may due to the enhanced hydrogen barrier property by less-developed hydrophilic phase. The best performing random BPSH with the IEC ~ 1.9 meq/g shows better performance (5.3 A/cm2) than the similarly thick (~50 μm) Nafion 212 (4.8 A/cm2) at 1.9 V, but the higher degradation rate (951 μV/h) than Nafion 212 (613 μV/h) at an accelerated stress test with the 360 consecutive alternating current densities of 3 and 0.02 A/cm2, simulating on and off a water electrolyzer.

Densely Quaternized Fluorinated Poly(fluorenyl ether)s with Excellent Conductivity and Stability for Vanadium Redox Flow Batteries.

Cationic group distribution and elemental composition are two key factors determining the conductivity and stability of anion exchange membranes (AEMs) for vanadium redox flow batteries (VRFBs). Herein, fluorinated tetra-dimethylaminomethyl-poly(fluorenyl ether)s (TAPFE)s were designed as the polymer precursors, which were reacted with 6-bromo-N,N,N-trimethylhexan-1-aminium bromide to introduce di-quaternary ammonium (DQA) containing side chains. The resultant DQA-TAPFEs with a rigid fluorinated backbone and flexible multi-cationic side chains exhibited distinct micro-phase separation as probed by small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM). DQA-TAPFE-20 with an ion exchange capacity (IEC) of 1.55 mmol g-1 exhibited a SO42- conductivity of 10.1 mS cm-1 at room temperature, much higher than that of a control AEM with an identical backbone but spaced out cationic groups, which had a similar IEC of 1.60 mmol g-1 but a SO42- conductivity of only 3.2 mS cm-1. Due to the Donnan repulsion effect, the DQA-TAPFEs exhibited significantly lower VO2+ permeability than Nafion 212. The VRFB assembled with DQA-TAPFE-20 achieved an energy efficiency of 80.4% at 80 mA cm-1 and a capacity retention rate of 82.9% after the 50th cycling test, both higher than those of the VRFB assembled with Nafion 212 and other AEMs in the literature. Therefore, the rationally designed DQA-TAPFEs are promising candidates for VRFB applications.

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

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

Side-chain manipulation of poly (phenylene oxide) based anion exchange membrane: Alkoxyl extender integrated with flexible spacer

Anion exchange membranes (AEMs) are an important component in anion exchange membrane fuel cells, and their side chain structures have a significant impact on membrane performance. Here, we construct a series of poly(phenylene oxide) (PPO)-based AEMs with varied side chain structures and investigate the effects of alkyl or alkoxyl extenders (or “tail” of the cation) and spacers (or the link between backbone and cation head group) on AEM performance. Our investigations demonstrate that the simultaneous introduction of alkoxyl extender and spacer can significantly improve water absorption and ion conductivity of membrane; this is attributed to the good hydrophilicity of alkoxyl group and flexibility of spacer that benefit the formation hydrophilic domains. At an IEC (ion exchange capacity) of 1.13 mmol/g, the AEM containing both spacer and alkoxyl extender exhibits a hydroxide conductivity of 87.3 mS/cm (80 °C), which is much higher than that of other spacer-containing membranes at similar IECs. The H2/O2 single cell constructed with this membrane exhibits a good peak power density of 292 mW/cm2 at 60 °C. In addition, the steric hindrance of the extender gives rise to higher alkali stability of the AEM than that of the membrane which does not have the extender. Our work provides a meaningful direction of side chain design which can effectively improve conductivity and alkaline stability of AEMs.

Comparison of Novel 1,1-Diphenylethylene Alternating Copolymer and Polystyrene Based Anion Exchange Membranes

This research investigates 1,1-diphenylethylene as a monomer in polymeric ion exchange membranes through the synthesis of a novel alternating copolymer functionalized for ion transport and analysis of mechanical and conductive properties. This will further the development of cost effective and high-performance ion exchange membranes, enabling energy systems that employ ion-transporting membranes, like fuel cells and electrolyzers, to become more economically viable, which in turn would lead to cleaner and more sustainable energy production.The goal of the project is to develop an understanding of chemical structure-physical property relationships to make self-assembling ion-conducting channels in ionomers which have physical properties that prevent the excess swelling that leads to the mechanical failure of membranes. In this vein, anion exchange membranes (AEMs) based on 1,1-diphenylethylene were synthesized and their ion conducting properties and physical state were characterized. Poly(1,1-diphenylethylene-alt-butadiene) alternating copolymer was synthesized via anionic polymerization, then selectively hydrogenated to eliminate double bonds in the polymer backbone. The pendant aromatic rings of the selectively hydrogenated poly(1,1-diphenylethylene-alt-butadiene) were then subjected to quantitatively controlled functionalized by super acid catalyzed Friedel-Crafts bromoalkylation, then aminated with trimethylamine. AEMs spanning a wide range of ion exchange capacities (IECs), 0.39-1.99 milliequiv./gram (meq./g), were made. Conductivity increased with IEC and temperature. Polystyrene based AEMs of similar IECs, 0.38-2.14 meq/g, were synthesized as a standard to compare the 1,1-diphenylethylene based copolymers. The 1,1-diphenylethylene based copolymers had favorable conductivities compared to polystyrene membranes of similar IEC. The glass transition temperature, water uptake, and dimensional swelling were investigated to explain the difference in conductivities between the two species. Figure 1

How hydrophobic side chain design affects water cluster connectivity in model polymer electrolyte membranes: Linear versus Y-shaped side chains

Dissipative Particle Dynamics is used to model water clustering in ion exchange membranes. We consider polymers with hydrophobic backbone alternatingly grafted with short hydrophilic ([C]) and hydrophobic linear (A6) or Y-shaped side chains (ie. Ap [As][As]; p(s) = 6(0), 4(1), 2(2), 0(3)). At 16% water volume fraction water cluster connectivity (DMC(W), water diffusivity, and inter water domain distance increase with length p and average number of bonds (<NbondA−C>) separating A and nearest C beads. Exchange of the pendent A and C beads for (A [C]A [A6])10 to form (A [A]A [A5C])10 decreases water cluster connectivity. This is because the lower number of bonds between nearest C beads for (A [C]A [A6])10 forces them to join the same water domain. This cooperative mechanism also explains deviations from the increase of DMC(W) with <NbondA−C> for similar ion exchange capacity (IEC). At high IEC (A [Ax+1C])10 architectures favour good connected pores which is surpassed by the H-phobic side chain (A [C]A [Ax])10 architectures at low IEC.

Anion exchange membranes based on ionic polybenzimidazoles crosslinked by thiol-ene reaction

In this study, we report the synthesis and properties of new anion exchange membranes using m-polybenzimidazole (m-PBI) as polymer backbone attached with quaternary ammonium group and alkene group as side chains. These ionic m-PBIs were crosslinked by thermal thiol-ene reaction. The ion exchange capacity (IEC) values and crosslinking degrees were controlled independently by the amount of attached quaternary ammonium groups and alkene groups, respectively. m-PBI based AEMs with IEC from 1.51 to 2.50 mmol/g exhibit hydroxide conductivity of 14 to 58 mS/cm at 80 °C. With similar IEC values, crosslinked membranes absorb less water but show similar hydroxide conductivity and better dimension stability than uncrosslinked membranes. A crosslinked membrane with an IEC value of 2.46 mmol/g can retain 84% of its pristine hydroxide conductivity after being immersed in 1.0 M sodium hydroxide solution at 60 °C after 720 h. These membranes in hydrated condition also show mechanical robustness with tensile strength of 10.8 to 14.6 MPa. From the high hydroxide conductivity, enhanced alkaline stability and good mechanical properties, crosslinked ionic m-PBIs are promising candidate as anion exchange membranes for fuel cell application.

Fluorinated poly(fluorenyl ether)s with linear multi-cationic side chains for vanadium redox flow batteries

The cationic group distribution along the polymeric backbones of anion exchange membranes (AEMs) has significant influence on their microscopic morphology and anion conductivity. To develop high-performance AEMs for vanadium redox flow batteries (VRFBs), a series of poly (fluorenyl ether) samples bearing di- and tri-quaternary ammonium side chains with similar ion exchange capacities (IECs) were synthesized by grafting cationic alkyl chains with tertiary amine-containing poly(fluorenyl ether) precursors. The experimental results indicate that the introduction of the multi-cationic side chains facilitates the formation of micro-phase-separated morphologies and enhances anion conductivity. Moreover, the number of spacer atoms between the quaternary ammonium groups on the side chains affects the water uptake of the membranes, thus complicating the relationship between the density of cationic group distribution and anion conductivity. The poly(fluorenyl ether)s with dicationic side chains and six spacing atoms (DQA-PFE-C6) showed the highest anion conductivity. A VRFB assembled with DQA-PFE-C6 exhibited a maximum power density of 239.80 mW cm−2 at 250 mA cm−2, which is significantly higher than a VRFB assembled with Nafion 212. Therefore, side chain engineering is an effective chemical approach to enhance the properties of AEMs for VRFB applications.

Self-crosslinked blend alkaline anion exchange membranes with bi-continuous phase separated morphology to enhance ion conductivity

Combining two facile methods, spinodal blending and self-crosslinking by chloromethyl groups are jointly utilized in fabricating alkaline anion exchange membranes (AEMs). Highly and moderately chloromethylated poly(ether ether ketone)s and sub-equimolar (to chloromethyl) amounts of 1-methylimidazole are mixed and reacted to form blend AEMs. Nanoscale bi-continuous phase separated morphologies are obtained due to spinodal decomposition. Subsequent heat treatment triggers self-crosslinking of the AEMs arising from residual chloromethyl groups in membranes. Compared with unblended and uncrosslinked AEMs having similar ion exchange capacity (IEC), the ion conductivity of the present AEMs is increased by 45.4% in water and 113% in 95% RH at 60 °C. Dimensional and alkaline stabilities of AEMs are also enhanced by the self-crosslinking, i.e., the swelling ratio decreases by 50.3% at 60 °C and the residual conductivity increases by 26.3% after alkaline treatment in 1 M NaOH at 60 °C. Interestingly, individual spinodal blending or self-crosslinking are found ineffective in overcoming the trade-off between AEM conductivity and stability, while a combination of both methods leads to a simultaneous improvement in conductivity and membrane stability.