Anion Exchange Membranes Incorporating Multi N-Spirocyclic Quaternary Ammonium Cations via Ultraviolet-Initiated Polymerization for Zinc Slurry-Air Flow Batteries

Four anion exchange membranes (AEMs) incorporating multi N-spirocyclic quaternary ammoniums via an ultraviolet (UV)-curing method were prepared and tested as separators in zinc (Zn) slurry-air flow batteries (referred to here as ZSAFBs), an environmentally friendly, high specific energy, and low cost energy storage device. The multi N-spirocyclic cations chain was grafted onto the PPO backbone via a hexyl spacer and its chain length, and the thus membrane ion exchange capacity was tuned by varying the molar ratio between the hexyl diallyl ammonium grafted on PPO and the diallyl piperidinium monomer. This rationally designed 3D polymer results in a membrane with a well-developed phase separation morphology, as confirmed by atomic force microscopy, with a high hydroxide ion conductivity and low zincate ion diffusion coefficient, i.e., the PO-6CH2-3x membrane exhibited 19 mS/cm and 2.3 × 10-12 m2/s at 20 °C, respectively. The membranes exhibited great alkaline stability, with PPO-6CH2-3x showing less than 8% drop in ionic conductivity after 2 weeks of accelerated degradation test (1 M KOH, 60 °C). The ZSAFB cell assembled with the PPO-6CH2-3x membrane exhibited an excellent maximum power density (153 mW/cm2), surpassing those of the reported values, making it a very promising membrane material for rechargeable ZSAFBs.