Influence of highly flexible di(biphenyl)ethane units on the properties of poly(arylene piperidinium) anion exchange membranes

Abstract

Introducing angled arene units and short flexible alkyl segments into rigid aromatic heteroatom-free polymer backbones are efficient strategies to improve the performance of anion exchange membranes (AEMs). So far, only very few monomers increasing backbone flexibility in polymers prepared by polyhydroxyalkylations have been presented, often with limited reactivity and polymerizability. Here, we present the synthesis of a highly reactive monomer [m-di(biphenyl)ethane, mD] containing two angled biphenyl units bridged by an ethane link using a straightforward noble-catalyst-free reductive homocoupling reaction. A series of copolymers were produced by polyhydroxyalkylation involving p-terphenyl, N-methylpiperidone, and different concentrations of mD. Small angle X-ray scattering of the AEMs indicated enhanced ionic clustering with increasing mD content and backbone flexibility. Moreover, water and KOH (aq.) uptake, hydroxide conductivity and alkaline stability increased with the mD content. The hydroxide conductivity of an AEM containing 25% mD units reached 187 mS cm⁻¹ at 80 °C in water, and an AEM with 50% mD units exhibited a conductivity of 53 mS cm⁻¹ in 2 M KOH (aq.) solution. Using only simple nickel foam electrodes, the latter AEM reached a current density of >400 mA cm⁻² at 2.5 V without any cell optimization. In summary, this work demonstrates a convenient synthetic strategy to incorporate flexible units in rigid aromatic polymers, offering improved membrane properties and valuable insights into the design and optimization of advanced AEM materials.