Highly conductive and alkaline stable partially fluorinated anion exchange membranes for alkaline fuel cells: Effect of ammonium head groups

The effect of ammonium structure on partially fluorinated anion exchange membranes was investigated for alkaline fuel cell applications. Precursor copolymers tethered with bromopropyl side chains (PAF-C3-Br) were successfully synthesized and quaternized quantitatively with amines to obtain a series of QPAF-C3-XY membranes containing various ammonium head groups. The head groups (XY) included dimethylbutyl (DMB), dimethylhexyl (DMH), dibutylmethyl (DB), and heterocyclic N-methylpiperidinium (Pip). All of the membranes exhibited well-developed phase-separated morphology regardless of the size of the head groups as confirmed by TEM images. DFT calculations suggested that the hydrophilic domain sizes (determined from TEM images) were dependent on the polar surface area (PSA) rather than the molecular volume of the head groups. Among the membranes, QPAF-C3-Pip possessed the most balanced properties of high conductivity, low water absorbability, reasonable mechanical strength, and alkaline stability. QPAF-C3-Pip membrane (IEC = 1.26 meq/g) achieved 232 mW/m² of the maximum power density in alkaline fuel cell, which was slightly higher than that of our previous QPAF-C3-TMA with typical trimethyl ammonium head groups (224 mW/cm²). In the durability test for 240 h, the voltage decreased from 0.74 V to 0.43 V with average decay of 1.29 mV/h. The post-durability NMR analysis revealed that the majority of the voltage loss was related to the catalyst layers.