Macromolecules, 2017, vol 50, 7, pp. 2784-2793
DOI:10.1021/acs.macromol.7b00168
Abstract
The alkaline stability of organic cations tethered to anion-exchange membranes (AEMs) is essential for the long-term performance of alkaline membrane fuel cells and electrolyzers. Here, we have prepared and studied the thermal and alkaline stability of a series of polyelectrolytes functionalized with N-spirocyclic quaternary ammonium (QA) cations. N,N-Diallylazacycloalkane quaternary salts were readily synthesized by diallylation of pyrrolidine, piperidine, azepane, and morpholine. These monomers were employed in radical-initiated cyclo-polymerizations to obtain the target poly(N,N-diallylazacycloalkane)s. 1H NMR spectroscopy revealed that the stability of the polyelectrolytes in 2 M KOD/D2O solutions critically depended on the ring size and the absence of additional heteroatoms in the ring. Thus, poly(N,N-diallylpiperidinium) showed the highest alkaline stability, with only minor signs of degradation at 120 °C after 14 days, while the polyelectrolytes based on the morpholine and azepane rings clearly degraded via both Hofmann elimination and ring-opening substitution already at 90 °C. Cross-linked water nonsoluble AEMs were prepared by copolymerizing N,N-diallylpiperidinium chloride with methylbenzyldiallylammonium groups tethered to poly(phenylene oxide). These transparent and mechanically robust AEMs reached high OH– conductivities, above 0.1 S cm–1 at 80 °C. The present work demonstrates the high alkaline stability of suitably configured N-spirocyclic QA cations, which will open up new prospects for readily accessible high performance polyelectrolytes and membranes.
