(28d) Saturated N-Heterocyclic Cationic Polymers As Anion Exchange Membranes in Alkaline Fuel Cells

Alkaline fuel cells (AFCs) hold the promise of a low-cost (no platinum) energy source for electric vehicles with no point-of-use carbon emissions. For this promise to be realized, AFCs require solid-state anion exchange membranes (AEMs) or separators that possess high chemically stability in alkaline conditions and also possess additional multiple desired properties of high hydroxide ion conductivities, robust mechanical properties, and are low in cost, easily processable, and can easily be integrated into AFCs. Polymers containing unsaturated N-heterocyclic cations (i.e., imidazolium) have been synthesized and investigated for various applications, including alkaline fuel cells. Recently, results have shown higher alkaline chemical stability for polymers bearing a saturated N-heterocyclic cation (pyrrolidinium) versus an unsaturated N-heterocyclic cation (imidazolium). Additionally, several studies on small molecules have reported increasing stability with increasing saturated N-heterocyclic cation size. However, to date, few studies have systematically investigated polymers containing saturated N-heterocyclic cations, specifically N-heterocyclic cations with larger ring sizes (i.e., azepanium, azocanium, azonanium). In this study, styrene-based saturatedN-heterocyclic cationic (SNHC) homopolymers with various covalent attached cations (methylpyrrolidinium, methylpiperidinium, methylazepanium, methylazocanium) were successfully synthesized to investigate the influence of ring size/strain on the properties of SNHC polymers. Alkaline chemical stability, temperature and humidity dependent water sorption, and ion conductivity of these SNHC polymers were measured and evaluated in regards to the relationship with ring size/strain. Additionally, multiblock polymers containing styrene-based saturated N-heterocyclic cationic chemistry were synthesized and investigated as functional polymer membranes. This work provides a fundamental understanding of the impact of the ring size/strain in SNHC polymers and furtherintroduces a promising chemistry for producing highly stable, ion conductive solid-state AEMs for AFCs.