Anion-conductive poly(2,6-dimethyl-1,4-phenylene oxide) grafted with tailored polystyrene chains for alkaline fuel cells

Abstract A series of polystyrene (PS)-grafted poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) having pendent quaternary ammonium groups were synthesized as anion exchange membranes (AEMs) by the “grafting onto” method via a combination of atom transfer radical polymerization (ATRP) and Cu(I)-catalyzed click chemistry. The length of PS grafting chains was controlled readily during ATRP. As expected, the polystyrene grafting chains showed excellent miscibility with PPO backbone. Therefore, transparent, flexible and tough membranes were obtained by solution casting. The miscible PS grafting chains induced well-defined hydrophobic-hydrophilic separation of the as-obtained PPO AEMs as confirmed by small-angle X-ray scattering (SAXS) technology. Moreover, the hydrophobic grafting chains can effectively control the water absorption, and thus improve the dimensional stability of AEMs in water. The PS-grafted AEMs showed higher IEC-normalized hydroxide conductivity but lower water uptake than the typical AEM without PS side chains, which may be attributed to the well-defined micro-phase separation in AEMs. The highest hydroxide conductivity of 15.9 mS/cm was achieved at 20 °C in spite of its low IEC value of 1.21 meq./g. Alkaline stability testing in 1 M NaOH at 80 °C demonstrated that PS-grafted PPO AEMs with side-chain-type QA cations showed excellent alkaline stability as evidenced by the change of hydroxide conductivity and the 1 H NMR analysis after 500 h testing. Further H 2 /O 2 alkaline fuel cell using PS-grafted PPO AEMs showed the maximum power density of 64.4 mW/cm 2 at a current density of 140 mA/cm 2 , which is much higher than that of typical AEMs with C-16 alkyl grafting chains.