Poly(N,N-diallylazacycloalkane)s for Anion-Exchange Membranes Functionalized with N-Spirocyclic Quaternary Ammonium Cations

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 radicalinitiated cyclo-polymerizations to obtain the target poly(N,Ndiallylazacycloalkane)s. 1 H NMR spectroscopy revealed that the stability of the polyelectrolytes in 2 M KOD/D 2 O 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.

Poly(arylene alkylene)s with pendant N-spirocyclic quaternary ammonium cations for anion exchange membranes

“…15 These results have inspired the preparation and study of a range of different AEMs functionalised with piperidinium [25][26][27][28][29] and spirocyclic QA cations. 13,[30][31][32][33][34][35] For example, we recently prepared poly (arylene N,N-dimethylpiperidinium)s by polycondensation of N-methyl-4-piperidone and electron-rich phenyl monomers via superelectrophilic activation in triic acid, followed by quaternisation reactions with iodomethane. 26 AEMs based on these polymers showed no structural degradation aer storage in 2 M NaOH at 60 C aer 360 h and a mere 5% ionic loss at 90 C aer the same period of time.…”

Ether-free polyfluorenes tethered with quinuclidinium cations as hydroxide exchange membranes

“…5,6 The vast number of studies on AEMs based on different aromatic polymer backbones, including polyethers, polysulfones, polyphenylenes, polybenzimidazoles, etc., has shown that aryl ether links are sensitive to hydrolysis under alkaline conditions, especially if this reaction is activated by nearby electron-withdrawing groups such as sulfone links. [9][10][11][12][13][14][15][16][17][18][19][20] Hence, AEMs based on ether-free aromatic backbones, such as polyphenylenes, 9,[21][22][23] poly(arylene alkylene)s [24][25][26][27][28][29][30][31][32] and sterically protected polyimidazoliums, [33][34][35][36] as well as on aliphatic backbones such as poly(diallyldialkyl ammonium) 37 and polynorbornenes, 38,39 have shown excellent alkaline stability.…”

“…42 The high stability of the alicyclic QAs has been rationalized by a low ring strain combined with the constrained conformations imposed by the ring structure, which increase the transition state energy of both substitution and elimination reactions. 42 Later studies by us and others on AEMs functionalized with alicyclic cations such as piperidinium 25,26,44 and different spirocyclic QAs 32,37,[45][46][47][48][49][50][51] have conrmed a high alkaline stability. However, the results also indicate that the stability is highly dependent on precisely how the cations are attached to the polymer backbone.…”

Cross‐Linked Spirocyclic Quaternary Ammonium‐Based Anion Exchange Membrane with Tunable Properties for Fuel Cell Applications

“…The excellent alkaline stability of spirocyclic QA cation was also confirmed by theoretical calculations . Based on their determination of alkaline stability of the small molecules, the aromatic polymers with pendent spirocyclic QA and piperidinium cations have been synthesized and used to prepare AEMs . Hickner's work indicated that the backbone of AEMs have effect on the alkaline stability of cations, and the AEM has aromatic backbone with ether groups is not stable in alkaline condition.…”

“…[34] Based on their determination of alkaline stability of the small molecules, the aromatic polymers with pendent spirocyclic QA and piperidi-nium cations have been synthesized and used to prepare AEMs. [34][35][36][37][38][39][40][41][42][43] Hickner's work indicated that the backbone of AEMs have effect on the alkaline stability of cations, [44] and the AEM has aromatic backbone with ether groups is not stable in alkaline condition. To date, there is few work on spirocyclic QAbased AEMs with an aliphatic backbone, and to understand the properties of this type of AEMs, a systematic study is needed.…”