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

Abstract

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–/SO4 2– 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–/SO4 2– 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.

Is It Possible to Prepare a “Super” Anion-Exchange Membrane by a Polypyrrole-Based Modification?

“…In the literature, there are a number of studies aimed at increasing the selectivity of IEMs in relation to the counterion transport both by synthesizing new membranes [ 19 , 20 ] and by modifying commercially available ones [ 21 , 22 , 23 , 24 ]. Attempts to increase selectivity often lead to a decrease in IEM conductivity [ 25 ].…”

Cross-Linked, Monovalent Selective Anion Exchange Membrane: Effect of Prealkylation and Co-ions on Selectivity

“…In the approaches mentioned above, the hydrophilic–hydrophobic balance was controlled either by varying the length of the alkyl group attached to the quaternary ammonium moiety or by increasing the alkyl chain length of the spacer used for cross-linking. ,, Another way to achieve maximum P SO 4 2– Cl – is by controlling the size of the ion sieve in the membrane matrix . Herein, our approach is to control the size of the ion channel and water content or hydrophilic–hydrophobic balance in the membrane by partial quaternization of a copolymer followed by cross-linking using another functional copolymer.…”

“…. 33 Irfan et al prepared a series of MASMs by mixing bromomethylated polyphenylene oxide with tertiary amines containing different alkyl chain lengths that show a P SOd 4 2− Cld − value of 13.07. 27 Liao et al prepared MASMs by introducing both the cationic and anionic groups in the membrane surface based on a long alkyl chain imidazole functionalized polymer followed by cross-linking.…”

“…Chen et al prepared MASMs with a rigid benzimidazole structure for monovalent and bivalent ion separation. They used poly­(arylene ether sulfone) having benzimidazole moiety followed by grafting to the imidazolium-based side chains to control the ion channel size as well as P SO 4 2– Cl – . Irfan et al prepared a series of MASMs by mixing bromomethylated polyphenylene oxide with tertiary amines containing different alkyl chain lengths that show a P SO 4 2– Cl – value of 13.07 .…”

Functionalized POSS-Modified SEBS-Based Composite Anion-Exchange Membranes for AEMFCs

“…To assess the feasibility of our proposal set forth in Figure , functionalized Pi-POSS was prepared through decorating POSS nanoparticles with N -methylpiperidium groups, which were demonstrated to possess distinguished resistance against nucleophilic substitution and elimination under high temperatures and alkaline conditions. AEMs consist of the hydrophobic polymer backbone and hydrophilic cationic groups, which directly determine the dimensional stability, alkaline stability, and ion conductivity of AEMs. − To improve the durability under alkaline conditions of AEMs, numerous studies on cationic groups and polymer backbones have been carried out. , Marino et al systematically investigated the alkaline stability of different kinds of quaternary ammonium (QA) groups . The results showed that the piperidinium-based cation certificates strong resistance against nucleophilic substitution and elimination under high temperatures and alkaline conditions …”