Structurally Well-Defined Anion-Exchange Membranes Containing Perfluoroalkyl and Ammonium-Functionalized Fluorenyl Groups

Abstract

Novel anion-conductive polymers containing perfluoroalkyl and ammonium-functionalized fluorene groups were synthesized and characterized. The quaternized polymers synthesized using a dimethylaminated fluorene monomer had a well-defined chemical structure in which each fluorenyl group was substituted with two ammonium groups at specific positions. The resulting polymers had a high molecular weight ( M n = 8.9–13.8 kDa, M w = 13.7–24.5 kDa) to provide bendable thin membranes with the ion-exchange capacity (IEC) ranging from 0.7 to 1.9 mequiv g –1 by solution casting. Both transmission electron microscopy images and small-angle X-ray scattering patterns suggested that the polymer membranes possessed a nanoscale phase-separated morphology based on the hydrophilic/hydrophobic differences in the polymer components. Unlike typical anion-exchange membranes found in the literature, hydroxide ion conductivity of the membranes did not increase with increasing IEC because of their high swelling capability in water. The membrane with IEC = 1.2 mequiv g –1 showed balanced properties of high hydroxide ion conductivity (81 mS cm –1 at 80 °C in water) and mechanical strength (>100% elongation and 14 MPa maximum stress at 80 °C, 60% relative humidity). The polymer main chains were stable in 4 M KOH for 1000 h, whereas the trimethylbenzyl-type ammonium groups degraded under the conditions to cause loss in the hydroxide ion conductivity. An H 2 /O 2 fuel cell with the membrane with IEC = 1.2 mequiv g –1 exhibited a maximum power density of 242 mW cm –2 at 580 mA cm –2 current density.

Rational molecular design of anion exchange membranes functionalized with alicyclic quaternary ammonium cations

“…Polymer backbone degradation in AEMs has primarily been observed to occur via aryl ether bonds cleavage, [13][14][15][16][17][18] motivating a recent change in research focus towards aryl-ether free backbones 19 such as polynorbornenes, [20][21][22][23][24] poly(arylene alkylene)s [25][26][27][28][29][30][31][32] and polystyrenes. [33][34][35][36][37][38] A more serious issue is the alkaline stability of the tethered cationic groups.…”

Effects of the N-alicyclic cation and backbone structures on the performance of poly(terphenyl)-based hydroxide exchange membranes

“…[14][15][16][17][18][19][20] Hence, there is now a clear trend in the eld to employ ether-free polymer backbones in the structural design of HEMs. 13,[21][22][23][24][25][26] In addition, the hydroxide ion is liable to react with QA cations which results in a loss of ion exchange capacity (IEC) and ionic conductivity. 5,[27][28][29][30] Depending on the structure of the cation, degradation may occur via, e.g., direct nucleophilic substitution by attack on an a-carbon, Hofmann belimination and different rearrangement reactions.…”

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.…”