As-prepared Anion Exchange Membranes Research Articles

Subnanometer Ion Channel Anion Exchange Membranes Having a Rigid Benzimidazole Structure for Selective Anion Separation.

Ion-conductive polymers having a well-defined phase-separated structure show the potential application of separating mono- and bivalent ion separation. In this work, three side-chain-type poly(arylene ether sulfone)-based anion exchange membranes (AEMs) have been fabricated to investigate the effect of the stiffness of the polymer backbone within AEMs on the Cl-/NO3- and Cl-/SO42- separation performance. Our investigations via small-angle X-ray scattering (SAXS), positron annihilation, and differential scanning calorimetry (DSC) demonstrate that the as-prepared AEM with a rigid benzimidazole structure in the backbone bears subnanometer ion channels resulting from the arrangement of the rigid polymer backbone. In particular, SAXS results demonstrate that the rigid benzimidazole-containing AEM in the wet state has an ion cluster size of 0.548 nm, which is smaller than that of an AEM with alkyl segments in the backbone (0.760 nm). Thus, in the electrodialysis (ED) process, the former exhibits a superior capacity of separating Cl-/SO42- ions relative to latter. Nevertheless, the benzimidazole-containing AEM shows an inability to separate the Cl-/NO3- ions, which is possibly due to the similar ion size of the two. The higher rotational energy barrier (4.3 × 10-3 Hartree) of benzimidazole units and the smaller polymer matrix free-volume (0.636%) in the AEM significantly contribute to the construction of smaller ion channels. As a result, it is believed that the rigid benzimidazole structure of this kind is a benefit to the construction of stable subnanometer ion channels in the AEM that can selectively separate ions with different sizes.

Clustered piperidinium-functionalized poly(terphenylene) anion exchange membranes with well-developed conductive nanochannels

Anion exchange membrane fuel cells (AEMFCs) attract considerable attention owing to their high-power density and potential utilization of cheap non-noble metal catalysts. However, anion exchange membranes (AEMs) still face the problems of low conductivity, poor dimensional and chemical stability. To address these issues, AEMs with clustered piperidinium groups and ether-bond-free poly(terphenylene) backbone (3QPAP-x, x = 0.3, 0.4, and 0.5) were designed. Transmission electron microscope results show that the clustered ionic groups are responsible for fabricating well-developed conductive nanochannels and restraining the swelling behavior of the membranes. 3QPAP-0.4 and 3QPAP-0.5 AEMs exhibit higher conductivity (117.5 mS cm−1, 80 °C) and lower swelling ratio than that of commercial FAA-3-50 (80.4 mS cm−1, 80 °C). The conductivity of 3QPAP-0.5 only decreased by 10.4% after treating with 1 M NaOH at 80 °C for 720 h. The Hofmann elimination degradation of the cationic groups is restrained by the long flexible alkyl chain between cations. Based on the high performance of 3QPAP-0.5, an H2-O2-type AEMFC reaches 291.2 mW cm−2 (60 °C), which demonstrates that the as-prepared AEMs are promising for application in fuel cells.

End-group crosslinked hexafluorobenzene contained anion exchange membranes

A kind of anion exchange membranes (AEMs) with CC bond end-group crosslinked structure was synthesized successfully. Unlike the traditional aliphatic AEMs, the AEMs were prepared in this work by a strategy to realize the CC bond thermal end-group crosslinking reaction, exhibiting an obvious microphase separation structure and a suitable dimensional stability. The well-defined ion channels constructed in the AEMs guarantee the fast OH− conduction, as confirmed via physical and chemical characterization. The conductivity was dramatically enhanced due to the effective ion channels and increased ion exchange capacity. Among the as-prepared AEMs, the PHFB-VBC-DQ-80% AEM has a conductivity of 135.80 mS cm−1 at 80 °C. The single cell based on PHFB-VBC-DQ-80% can achieve a power density of 141.7 mW cm−2 at a current density of 260 mA cm−2 at 80 °C. The AEMs show good thermal stability verified by a thermogravimetric analyzer (TGA). Furthermore, the ionic conductivity of PHFB-VBC-DQ-80% only decreased by 7.1% after being soaked in a 2 M NaOH solution at 80 °C for 500 h.

Monovalent anion selective anion-exchange membranes with imidazolium salt-terminated side-chains: Investigating the effect of hydrophobic alkyl spacer length

Ion-conductive polymers having highly ordered and well-defined phase-separated structures are potential materials for use as monovalent anion-selective membranes (MASMs). In this work, to investigate the effect of hydrophobic spacers, four side-chain-type imidazolium salt-tethered poly(arylene ether sulfone) anion-exchange membranes (AEMs) with different alkyl spacer lengths (–CH2– number: 3, 6, 9, and 12) were fabricated for use in electrodialysis (ED). Our investigations using atomic force microscopy, small angle X-ray scattering and X-ray diffraction demonstrate that the extended alkyl spacers change the nano-phase-separated structures, with an increased ion cluster space ranging from 3.29 nm to 7.76 nm, resulting from increased immiscibility between the additional hydrophobic alkyl spacers and (i) the hydrophobic aromatic backbones and (ii) the hydrophilic ion-conductive groups. In addition, with the extension of the alkyl spacer, chloride (Cl−) ion conductivity at 25 °C decreases from 16.4 mS cm−1 to 11.4 mS cm−1 and water uptake is reduced to 12.8% (20 °C). It is found that the as-prepared AEM with a hexyl alkyl spacer shows the superior perm-selectivity (Cl−/SO42−) of 7.10 (separation efficiency: 70.3%), as compared with three others (3.48, 6.81, and 4.26). This work thus presents an efficient strategy to guide the architectural design of novel MASMs.

Anion exchange membranes with dense N-spirocyclic cations as side-chain

To prepare anion exchange membranes (AEMs) with high performances, we tethered the as-synthesized oligomers with dense N-spirocyclic cations (NS) as the side-chain to poly(ether sulfone) (PES). The steric hindrance of spirocyclic structure and electron-donating effect of alkyl spacers significantly enhance the stability of cations in harsh alkaline and high hydration conditions. Correspondingly, the ionic exchange capacity (IEC) of PES–NS–10% only decreased by 13.8% and the conductivity remained 84.3% after being immersed in harsh environment (80 °C, 2 M NaOH) for 864 h. In addition, the high density of N-spirocyclic cations in the side-chain benefits the formation of clear microphase separation, as confirmed by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The highest conductivity of 95.5 mS cm-1 was achieved by PES–NS–10% at 80 °C while the corresponding swelling ratio was only 7.69%. Furthermore, the single cell assembled with PES–NS–10% showed a peak power density of 110.1 mW cm-2 at 300 mA cm-2. Durability test of the single cell based on PES–NS–10% under 100 mA cm-2 also demonstrates that the as-prepared AEMs with NS have potential application for fuel cells.

Heterogeneous anion-exchange membrane: Influences of charged binders with crosslinking structure on electrodialytic performance

It has practical significance to improve the electrodialytic performance of heterogeneous ion-exchange membrane. In this work, appropriate amount of N,N-dimethylaminoethyl methacrylate (DMAEMA) was added into conventional casting solution composed of poly(vinylchloride), anion-exchange resin particles and N,N-dimethylformamide. After the polymerization of DMAEMA and solvent evaporation, the membrane precursor was formed and then treated by p-xylylene dichloride for the simultaneous quaternization and crosslinking of PDMAEMA. Subsequently, the effects of charged binders with a semi-interpenetrating crosslinking structure on the electrochemical properties of anion-exchange membranes (AEMs) were investigated. Above all, the evolution of chemical composition and morphology changes were recorded by FTIR and SEM, respectively. Moreover, the novel membranes were evaluated from different perspectives, such as Water uptake, Fixed ion concentration, Area electrical resistance, Transport number of counter-ion, Salt permeability coefficient, Limiting current and compared with those of the conventional heterogeneous AEMs with a similar ion-exchange capacity. At last, as-prepared AEMs were applied in simulated electrodialysis experiments. Results showed that the novel membrane-preparing technology made the heterogeneous AEM look and behave more like homogeneous one. Of course, the scheme is also suitable to the hot-pressing technology for the preparation of heterogeneous AEM.

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.

Development of acid block anion exchange membrane by structure design and its possible application in waste acid recovery

Abstract Nowadays, excess quantities of industrial waste acid with low concentration have become not only a serious threat to environment but also a huge resource waste. Electrodialysis (ED) should have played important roles therein. However, the substantial proton leakage of anion exchange membrane (AEM) seriously deteriorates its work performances. In this report, the water uptake of AEM is controlled for alleviating the problem by material selection, for example, hydrophobic skeleton and weakly basic anion exchange groups. Of note, two kinds of membrane structures are designed and investigated. One is the semi-interpenetrating network which is achieved by blending PVDF with DMAEMA and DVB copolymer, the other is a so-called microphase-separated structure which is driven by the polarity difference of segments of the side-chain-type membrane material, PVDF-g-PDMAEMA. Series of electrodialysis experiments confirm that the as-prepared AEMs do exhibit remarkable acid block property. In particular, PVDF-g-PDMAEMA AEM displays more outstanding comprehensive properties, such as area electrical resistance, anti-swelling and limiting current densities. Based on comparison of results achieved in electrochemical characterizations, morphology observations and measurements of water uptake and acid adsorption, it is concluded that the microphase separation of the hydrophilic and hydrophobic segments contributes to the formation of ion nanochannels and then induces some consequential confinement effects.

Effect of Fluorene Groups on the Properties of Multiblock Poly(arylene ether sulfone)s-Based Anion-Exchange Membranes

Two kinds of novel multiblock poly(arylene ether sulfone)s were synthesized via block copolycondensation of telechelic oligomers as a starting material for the preparation of anion-exchange membranes (AEMs). The as-synthesized copolymers have extremely similar main chains. The difference is that the benzylmethyl groups for one are located on the fluorene-sulfone segments and they are located on the isopropylidene-sulfone segments for the other. The benzylmethyl moieties served as precursors to cationic sites and were brominated using N-bromosuccinimide (NBS) and then quaternized with N,N,N',N'-tetramethyl-1,6-diaminohexane (TMHDA). Controlled bromination and quaternization at specific positions of the benzylmethy-containing fluorene-sulfone segments and isopropylidene-sulfone segments can be achieved. 1H NMR spectroscopy, Fourier transform infrared spectroscopy, and gel permeation chromatography were used to characterize the as-synthesized copolymers. Distinct microphase separation in the as-prepared AEMs was observed using small-angle X-ray scattering and transmission electron microscopy. The AEM containing fluorene-sulfone segments (IEC=1.89 meq·g(-1)) showed higher ionic conductivity and methanol permeability than that containing isopropylidene-sulfone segments (IEC=2.03 meq·g(-1)). Moreover, the former showed better alkaline stability than the latter.