Quaternized poly(arylene ether) ionomers containing triphenyl methane groups for alkaline anion exchange membranes

Abstract

A series of hydroxide ion conductive poly(arylene ether) ionomers containing functionalized triphenyl methane groups and various backbones were prepared via polycondensation, chloromethylation, quaternization, and subsequent alkalization reaction. The structures of the poly(arylene ether)s (PAEs), chloromethylated poly(arylene ether)s (CMPAEs) and quaternized ammonium-substituted poly(arylene ether)s (QPAEs) were confirmed using ¹H NMR and FT-IR techniques. Thermal stabilities of the PAEs, CMPAEs and QPAEs were evaluated by thermal gravimetric analysis (TGA). The water uptakes, swelling ratios, ion exchange capacities (IECs), hydroxide conductivities and chemical stabilities of the membranes derived from the synthesized ionomers were assessed as anion exchange membranes. The IEC of the ionomers ranged from 0.57 to 2.59 mmol g⁻¹, which can be controlled by chloromethylation reaction conditions. The hydroxide conductivities of the QPAE membranes increase dramatically with increasing temperature. The QPAE-a membrane with an IEC value of 2.59 mmol g⁻¹ displayed the highest hydroxide conductivities of 14.9 and 84.6 mS cm⁻¹ at 20 °C and 80 °C, respectively. The hydroxide transport activation energy for the QPAE membranes ranged from 18.07 to 24.07 kJ mol⁻¹. The first degradation temperatures of the QPAE membranes were around 185 °C. The tensile strength varied from 18 to 41 MPa at 25 °C. The QPAE membrane retained 86% of its mechanical properties and 83% of its hydroxide conductivity after being conditioned with 1 M NaOH at 60 °C for 120 h. In particular, QPAE-a retains 87% of its original IEC under very harsh conditions of 6 M NaOH at 60 °C for 168 h. These properties of the ionomer membranes demonstrate their potential as anion exchange membranes for alkaline fuel cells.