Simultaneous, Synergistic Control of Ion Exchange Capacity and Cross-Linking of Sterically-Protected Poly(benzimidazolium)s

Abstract

The application of highly charged anion exchange membranes is often limited by strong water sorption leading to excessive swelling and eventual dissolution, especially at elevated temperatures. The cross-linking of polymers has been shown to be an excellent mitigation strategy but this often restricts membrane and ionomer processing methods. Here, we explore the reaction, stability, and utility of a cross-linking agent for the recently discovered class of cationic polymers: methylated, C2protected poly(benzimidazolium)s. In situ reaction and formation of p-xylyl cross-linking groups is found to provide novel, highly functionalized, hydroxide-stable membranes and films with enhanced anion conductivities and superior mechanical properties compared to un-cross-linked polymers. The versatility of this strategy will be important in the design of polymers for the preparation of stable, hydroxide-conducting films, membranes, and ionomers.

From polybenzimidazoles to polybenzimidazoliums and polybenzimidazolides

“…controlling the degree of N-methylation 92,94 or by crosslinking. 93 Furthermore, Ma et al 155 introduced an even more bulky p-phenylene link in the polymer, containing four phenyl substituents. The base polymer was obtained from randomcopolymerization with excess isophthalic acid (see Table 3; Entry 11), which implies that a large fraction of unprotected benzimidazolium C2 carbons are exposed to the hydroxide ions.…”

The effect of ionomer content in catalyst layers in anion-exchange membrane water electrolyzers prepared with reinforced membranes (Aemion+™)

“…No measurements were taken to ensure that the membranes and ionomer are fully in hydroxide form, a small amount of retained iodide is expected even aer two days in KOH, and carbonates may form whenever the materials are in contact with air aer ion exchange. 5,21,22 To improve handling, membranes were xed in PTFE templates with superimposed openings exposing the catalyst layers (see Fig. S2 †), which kept the CCMs at and avoids their curling, which may otherwise hinder successful assembly of the MEA.…”

Graphene oxide–polybenzimidazolium nanocomposite anion exchange membranes for electrodialysis

“…28 Polybenzimidazolium AEMs reported in the literature exhibited excellent IEC and conductivity. 29,30 These membranes have mostly been considered for anion exchange fuel cells, but their applicability in that field is limited due to stability problems of the hydroxide form. 31,32 The halide forms on the other hand exhibited good stability, which may be even further enhanced by GO filler.…”