High Ion Conductivity and Diffusivity in the Anion Exchange Membrane Enabled by Tethering with Multication Strings on the Poly(biphenyl alkylene) Backbone

Abstract

Aryl ether-free polyaromatics tethered with quaternary ammoniums (QAs) offer a promising candidate as an anion exchange membrane (AEM) for use in alkaline fuel cells. However, the traditional trade-off dilemma between the ion conductivity and water swelling of the AEM still remains. Herein, we report a series of poly(biphenyl alkylene) (PBPA) polymers tethered with multiple QAs per side chain that combine the advantages of a high ion conductivity, high ion diffusion coefficient, and suppressed water uptake. Upon increasing the number of QAs on each side chain, the degree of phase separation of the resulting AEM increased, thereby improving the ion diffusion coefficient from 2.03 × 10–6 cm2 s–1 for a single-QA-based PBPA100-1QA to 2.26 × 10–6 cm2 s–1 for a triple-QA-based PBPA35-3QA. The side chain triply-charged membranes show much improved performances. Specifically, at a given ion exchange capacity (IEC) of 2.5 mmol g–1, despite a notably low water uptake that was observed, the PBPA35-3QA exhibited a conductivity ∼40% higher than that of PBPA100-1QA. The highest conductivity was observed for PBPA50-3QA (IEC = 3.1 mmol g–1), achieving a 63 mS cm–1 chloride conductivity at 80 °C, while the corresponding water uptake was well-suppressed within 116 wt %. Owing to the high conductivity and ion diffusivity as well as the suppressed water uptake, a fuel cell fabricated with the 25 μm thick PBPA35-3QA membrane showed a low high-frequency resistance of 0.03 Ω cm2. However, the triple-QA-based membrane showed an 18% cation degradation after aging for 300 h in 2 M NaOH at 80 °C, which is a significantly inferior value than those of the counterparts with a low number of QAs on the side chains.