Effect of biphenyl groups on the properties of poly(fluorenylidene piperidinium) based anion exchange membranes for applications in water electrolyzers†

A series of quaternized poly(arylene fluorenylidene piperidinium)-based copolymers were synthesized using different hydrophobic components, including biphenyl, m- or p-terphenyl, and 9,9-dimethylfluorenyl groups. Among them, the quaternized poly(biphenylene fluorenylidene piperidinium) had the best solvent solubility and membrane formability. Transmission electron microscopy showed that poly(biphenylene fluorenylidene piperidinium) (QBPh-Pip) had a well-interconnected nanoscale phase-separated morphology. The QBPh-Pip membrane with an ion exchange capacity of 1.9 mequiv. g⁻¹ exhibited the most balanced properties, with low water uptake (95% at 80 °C), low swelling (45%), and high hydroxide ion conductivity (160 mS cm⁻¹ at 80 °C). Despite the low water absorption, rapid ion mobility led to high ion conductivity, as calculated using normalized diffusion coefficients. Furthermore, the QBPh-Pip membrane exhibited excellent alkaline stability (91.5% (141 mS cm⁻¹) of the initial conductivity after 1000 h in 8 M potassium hydroxide at 80 °C) and excellent mechanical properties (29.0 MPa of maximum stress and 134% elongation at break). In a water electrolysis cell using a nickel iron oxide anode catalyst, the QBPh-Pip membrane achieved a low cell voltage (1.7 V at 1.0 A cm⁻²) with 72% efficiency. The QBPh-Pip cell was durable for 1000 h at a constant current density of 1.0 A cm⁻² with minor voltage decay of 70 μV h⁻¹.