N‑Functionalized Poly(biphenyl piperidine) Ion-Solvating Membranes for Alkaline Water Electrolysis

Alkaline ion-solvating membranes (ISMs) combine the high conductivity of aqueous hydroxide solutions with the gas barrier properties of dense polymer films and are, therefore, an attractive class of electrode separators for alkaline water electrolysis. The absence of intrinsically unstable cationic groups in these membranes broadens the scope of polymer chemistries that can be used compared with conventional anion exchange membranes. Exceptional cell performance has been reported with polybenzimidazole-based ISM, but the lifetime is limited due to polymer degradation through polymer chain scission. This work reports the synthesis and characterization of N-functionalized poly(biphenyl piperidine) (PBP)-based ISM. PBP has a full hydrocarbon backbone and has gained a lot of attention in the anion exchange membrane community due to its excellent alkaline stability. The pristine poly(biphenyl piperidine) is highly hydrophobic, and the electrolyte uptake is, therefore, low. To promote electrolyte uptake and to enhance conductivity, the piperidine moieties were functionalized with four different polar pendant groups via N-substitution, containing alcohol, primary amine, carboxylate, and sulfonate functionalities. The former two, which are not charged in the pH range investigated, were found to bring about negligible changes in electrolyte uptake and other key properties. The membranes functionalized with anionic moieties (carboxylate and sulfonate), on the other hand, allowed the increase of KOH electrolyte uptake to above 50%, and proved to be suitable for water electrolysis applications.