Properties and Morphologies of Anion-Exchange Membranes with Different Lengths of Fluorinated Hydrophobic Chains

Abstract

An anion-exchange electrolyte membrane, QPAF(C6)-4, polymerized with hydrophobic 1,4′-bis(3-chlorophenyl)perfluorohexane and hydrophilic (6,6′-(2,7-dichloro-9 H -fluorene-9.9-diyl)bis( N , N -dimethylhexan-1-amine) is physically flexible and chemically stable. The drawbacks are relatively large water swelling and lower OH – conductivity at higher water uptakes, which are considered to be due to the entanglement of the flexible hydrophobic structure of the membrane. In this study, a QPAF(C4)-4 membrane was newly synthesized with shortened hydrophobic fluoroalkyl chains. Unexpectedly, QPAF(C4)-4 showed a higher water uptake and a lower bulk/surface conductivity than QPAF(C6)-4 possibly due to the decrease in hydrophobicity with a smaller number of fluorine atoms. The thermal stability of QPAF(C4)-4 was higher than that of QAPF(C6)-4, possibly due to the rigidity of the QAPF(C4)-4 structure. A higher mechanical strength of QAPF(C6)-4 than that of QPAF(C4)-4 could be explained by the larger interactions between molecules, as shown in the ultraviolet–visible spectrum. The interactions of molecules were understood in more detail with density functional theory calculations. Both the chemical structures of the polymers and the arrangements of the polymers in the membranes were found to influence the membrane properties.

Development of highly conductive anion exchange membranes based on crosslinked PIM-SEBS with high free volume

“…This inhibits, rather than facilitates, microphase separation, ultimately leading to reduced mechanical properties and overall performance. 29,30 Thus, suppressing excessive WU and simultaneously achieving high ionic conductivity in polyarylene-type AEMs requires either inducing a structural modicationby introducing, graing, or hyperbranching spacer-type multi-cation conducting head groups, for exampleor adding physical or chemical crosslinkers. [20][21][22][31][32][33][34][35] The aforementioned approach, in which microphase separation between hydrophilic and hydrophobic domains is used to control the increase in WU and, thus, achieve improved mechanical and electrochemical properties, including ionic conductivity, is considered less suitable for polyarylene-based copolymers.…”

“…A simpler alternative for maximizing microphase separation between the hydrophilic and hydrophobic domains in polyarylene-based copolymers is to incorporate hydrophilic functional groups, such as ether linkers, into the polymer side chains or introduce uorine directly into the polyarylene structure as a hydrophobic group. 14,[29][30][31][36][37][38] In addition, ongoing research efforts address the use of polymer backbones with high free volume, combined with a rigid structure, to induce microphase separation in AEMs. [39][40][41][42][43][44][45][46][47] Free volume refers to the empty spaces between the polymer backbones due to the polymer components.…”

“…This inhibits, rather than facilitates, microphase separation, ultimately leading to reduced mechanical properties and overall performance. 29,30…”

“…A simpler alternative for maximizing microphase separation between the hydrophilic and hydrophobic domains in polyarylene-based copolymers is to incorporate hydrophilic functional groups, such as ether linkers, into the polymer side chains or introduce fluorine directly into the polyarylene structure as a hydrophobic group. 14,29–31,36–38…”

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 …”