Side-chain cation-grafted poly(biphenyl piperidine) membranes for anion exchange membrane fuel cells

To achieve high-performance anion exchange membranes (AEMs) with superior conductivity and stability, a series of novel poly(biphenyl piperidinium) (PBP-n-Pip) membranes are synthesized by tethering cationic groups to the backbone utilizing flexible hydrophobic alkyl spacers with various lengths. All of the membranes exhibit excellent film-forming ability and tensile strength (42.34–53.49 MPa). Well-defined microphase separation is confirmed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). The PBP-6-Pip membrane exhibits the conductivity of 117.1 mS cm−1 at 80 °C due to the most excellent microphase-separated morphology and highest water uptake. The long-term alkali resistance of AEMs can be improved by extending the length of the flexible alkyl chains appropriately. After immersion in 2 M NaOH at 80 °C for 1500 h, the initial conductivity of PBP-6-Pip with intermediate spacer chains and PBP-8-Pip with longer spacer chains both remains above 84%. Furthermore, the PBP-6-Pip membrane with the highest conductivity achieves a peak power density of 307 mW cm−2 at 80 °C in the H2/O2 single fuel cell. A long-term life test of this MEA shows a voltage decay rate of 2.87 mV/h over 40 h of operation under 0.2 A cm−2 at 60 °C.