Poly(biphenyl piperidine) anion exchange membranes with long hydrophilic side chains for water electrolysis

Anion exchange membranes (AEMs) are key core materials in water electrolysis, requiring both high conductivity and good dimensional stability and chemical durability. In this work, a series of poly(biphenyl piperidinium) AEMs with side octadecyl groups (Q18PBP-x) are prepared through the superacid catalyzed polymerization and grafting reactions. The long hydrophobic alkyl side chains attached onto the ether-free backbone effectively improve the dimensional stability, chemical durability and ionic conductivity of the obtained AEMs at high temperature. At 80 °C, the water absorption and swelling rate of Q18PBP-30 membrane with IEC of 2.28 mmol g⁻¹ are 47.8 and 10.8%, respectively. Its ionic conductivity is 79.4 mS cm⁻¹ and only decrease by 6.9% after 480 h of immersion in 2 M NaOH at 80 °C. The current density of water electrolyzer assembled with representative Q18PBP-30 membrane reaches 0.53 A cm⁻² in 2 M KOH at 2.1 V, and its voltage fluctuation of durability testing is only 15.8% after 480 h at the current density of 300 mA cm⁻². These results suggest that grafting long hydrophobic alkyl chains on the backbone of AEMs is a promising strategy to enhance their properties for water electrolysis.