Enhanced ionic conductivity and chemical stability of anion exchange membranes prepared from ether-free poly(biphenyl alkylene piperidinium) with alkyl spacers for water electrolysis

Abstract

A series of poly(biphenyl alkylene piperidinium) (PBAPip-n-OH) anion exchange membranes with outstanding conductivity and alkaline stability are synthesized by tethering piperidinium functional groups onto ether-free aromatic polymer backbones via mobile alkyl spacers. The inclusion of alkyl spacers facilitates the formation of hydrophilic ionic clusters distinctly separated from the hydrophobic matrix. The piperidinium groups anchored at the end of alkyl side chains enable the fast transport of hydroxide ions, while the rigid ether-free biphenyl backbones preserve the chemical, dimensional, and mechanical stability of the membrane. As the hydroxide conductivity of the PBAPip-n-OH membrane increases with increasing alkyl spacer length up to a certain level, PBAPip-6-OH demonstrates the highest conductivity of 65.14 mS cm⁻¹, maintaining the robust mechanical strength (tensile strength of 67.6 MPa) at 30 °C. This membrane also exhibits excellent chemical stability, showing a weight loss of less than 5% after 30 days of immersion in 1 M potassium hydroxide solution. Given these outstanding properties, the PBAPip-6-OH membrane is employed to water electrolysis cells with nickel foam and platinum on carbon serving as anode and cathode catalytic materials, respectively, where it shows a high current density of 2.8 A cm⁻² at 2 V and 60 °C.